"Half-sibling" is a term that refers to a child who shares only one biological parent with their siblings. Sometimes they're raised separately and sometimes half-siblings are raised together. Either, however, can easily cause problems in fiction.
A character feels troubled over the fact they're only half-siblings. Maybe they don't think they "really" count as a sibling, they don't look like their half-sibling, they think their step-parent prefers their biological children over them, or they're jealous of their half-siblings for getting more attention from their mutual parent. Either way, the character is distraught over being a half-sibling.
In some cases, characters aren't told that they're half-siblings until later in life. This can cause troubles of its own.
Compare Bastard Angst, where the half-sibling actually is treated poorly by the family and/or society as a whole because they were born out of wedlock, as opposed to them just having internal worries about it. This may lead to or result from Surprise Incest if the siblings are attracted to each other and haven't been informed of their biological connection. May be part of Blended Family Drama.
- Berserk: Serpico is the bastard son of a nobleman assigned as his half-sister's bodyguard. Whatever he feels about this (and the fact that he's obviously the most well-adjusted of the noble's three children), he keeps very well hidden, while Farnese once forced him to burn his senile mother as a witch to ensure her family was all he had left.
- Kazuto/Kirito from Sword Art Online experienced a similar thing when he finds out his sister was actually his cousin while his parents were really his aunt and uncle (his real parents having died a long time ago). Upon secretly discovering this, Kazuto became distant from his family and immersed himself in gaming as a form of escapism. He eventually does put this in the past and vows to get closer to his sister.
- The core of the Frozen fic The Queen of Hearts is Elsa finding out rumors that she's not biologically related to her dad and then trying to find out if the rumors are true. She spends a lot of time angsting about not being Anna's full-blood sister. She plans to hand the throne to Anna if the rumors are true. They are legit. Her parents couldn't conceive an heir so they got a mountain man to impregnate the queen. In the end, Elsa comes to terms with her parentage and decides that it doesn't matter what her biological relationship to her sister and father are.
- RWBY: While averted in canon, where half-sisters Yang and Ruby are close (it's not even explicitly mentioned that they're only half-sisters until volume 2), this is very common in fanfics.
- Inverted in the Dark Fic Hello Darkness, My Only Friend. Yang doesn't have an issue with being Ruby's half-sister, but Ruby uses it against Yang and calls her an "accidental older sister" in an argument.
- In Pawn to Queen, Yang is bitter over Summer not having been her biological mother. When Ruby tries to coax her sister to visit their mom's grave, Yang acts dismissive and ends with "She wasn't even my mom anyway". This causes Ruby to snap at her that Summer was the only mom she had and her biological mother Raven ran off because she didn't want her.
- Used in a roundabout way in To Claim A Rose. Ruby is a brunette while her half-sister and father are both blond. As a result, kids at school tease Ruby and say that her also-dark-haired uncle Qrow is her father. The idea of being a Chocolate Baby upsets Ruby a lot. Luckily, Qrow turns down the idea by noting she just takes after her mother.
- Ruby feels the "didn't know they were half-siblings" variety of this in RWBY: Scars. 15-year old Ruby had always thought she and her older sister Yang were full-blooded sisters. In chapter 29, Yang accidentally slips and mentions her mother Raven by name. Ruby wants to know who "Raven" is, so Yang gives in and tells her. This news makes Ruby upset, especially because Yang told their teammates before she even told her sister. It also confuses her how their dad could have been with two different women in a short period of time, making Summer feel like a rebound. Biologically, Ruby isn't even Yang's half-sister; she's her cousin but neither sister knows this.
- In RWBY Alternate, Yang is an Aloof Big Sister towards her younger half-sister because she's bitter over the Parental Favoritism Ruby gets from their father.
- Zira from The Future Of Our Past resented her half-brother Mheetu even prior to his birth. Scar rejected her as his heir because she was female and because she was born out of wedlock, yet he doted over the idea of a son to call his heir. Zira doesn't even like calling Mheetu her brother.
- In Zootopia A Tail Of Two Mayor Lionhearts Half-Sister, Lucy Ironclaw Lionheart resents her both her father for leaving her mother and her half-brother for existing.
- In Joseph: King of Dreams, Simeon calls Joseph "brother" during a fight, to which he angrily snaps "Half-brother!" This becomes an Ironic/Meaningful Echo later when they use it as a reason for selling him into slavery.note
Joseph: (being dragged away by slavers) I'm your brother! I'm your brother!Judah: (quietly) Half-brother.
- This is subtextually used in The Lion King II: Simba's Pride. Nuka is jealous towards his younger half-brother Kovu for having their mother's favour.
- Aquaman (2018) features half-brothers Arthur and Orm. Arthur, who was raised on land, resents what he considers his mother's abandonment when she returns to Atlantis rather than stay to raise him. This pain was compounded by the knowledge of his younger half-brother who lived with and was raised by Atlanna. However, when Arthur later discovers that his mother was exiled to a presumed death sentence for the crime of mating with a land-dweller and having a half-breed child, he wants nothing to do with Atlantis. Orm, who was raised in Atlantis and groomed to be king, hates that his mother's heart always truly longed to be with Arthur and his father Thomas. Furthermore, despite Orm being on the throne, he knows that Arthur, as Queen Atlanta's firstborn, is the rightful heir. Their resentment of each comes to a head when Orm decides to attack the land, which he claims is motivated by the land-dwellers' rampant pollution of the ocean, but his true motivation seems to be a burning desire to destroy his older brother's home as payback for stealing their mother's affection.
- Mathilda Lando in The Professional flat out states that she is not fully related to her step-mother and sister.
Mathilda: She's not my mother. My sister, she wanted to lose some weight anyway. But she never looked better. Not even my real sister.
- In A Song of Ice and Fire and its television adaptation Game of Thrones, this is one of Jon Snow's early defining traits. Despite being (supposedly) the bastard son of Eddard Stark, he has a fantastic relationship with all of his brothers and sisters, particularly with Robb and Arya, though his stepmother Catelyn Stark hates him, and he is ostracized from the family during important events, such as the arrival of King Robert Baratheon. Some advice from fellow outcast Tyrion Lannister helps him start to overcome this chip on his shoulder, but his desire to be a trueborn Stark is a major part of his character across all five books and five seasons of the show.
- In Warrior Cats, Brambleclaw and Tawnypelt are half-siblings with Hawkfrost and Mothwing through their father, Tigerstar, the Big Bad of the series. Brambleclaw spends a lot of time in the latter half of the second arc angsting over the fact that he and Hawkfrost (who are nearly identical to each other - and their father) are half-siblings: he desires to know him better as they are kin, but they're also from different Clans which makes that difficult. It's also questioned (especially by Brambleclaw's closest friend, Squirrelflight) how much Hawkfrost is influenced/inspired by their father's evil actions, which also brings up the question that if he can't be trusted, then whether Brambleclaw himself can be as well due to their shared blood.
- In Everworld, April and Senna are half-sisters; Senna was conceived through an extramarital affair, then moved in with April's family when the two were children. It's interesting to note that while April claims that her mother never treated Senna any differently than her, Senna's Villain Episode paints a different picture, with April clearly getting Parental Favoritism. Either way, the two absolutely hate each other, and the fact that Senna is a sociopath with magical powers obviously doesn't help.
- Played with in The Hate U Give, in that the half-sibling himself isn't the one causing the tension. The character Seven half-brother to both the main character, Starr (sharing her father) and her friend Kenya (sharing her mother). While the two girls usually get along, Kenya referring to Seven as, "My brother" when speaking to Starr annoys her as it feels like he's being claimed. By the end of the book, Kenya acknowledges she was wrong to do so and refers to him as, "Our brother."
- In Anne with an E Elijah is jealous upon discovering he has a newborn half-sister because she will have a better, easier life than him. He even refuses to hold her.
Thea: (Talking to Oliver) You're my half brother. And you know who else was my half brother? Tommy. Tommy, who I tried to kiss. I tried to kiss my half brother— before my ''real'' father killed him! That's how screwed up I am!
- In episode 2x19, "The Man Under The Hood" of Arrow, Thea has a minor freak out when she finds out that Tommy, her older brother's best friend that she crushed on throughout her childhood, was actually her older paternal half-brother.
Emiko:"I wanted to be a Queen..."
- In season 7, Oliver discovers Moira wasn't the only Queen parent with extramarital secrets. Robert had an affair that led to the birth of Emiko Queen, making her Oliver's half-sister. Following the murder of her mother, Emiko is emotionally in the same place as season 1 Oliver and she repeatedly rejects his attempts to connect with her due to her hurt over watching Oliver, her biological sibling, grow up in the lap of luxury while she and her mother struggled in the Glades, the most impoverished, crime-ridden part of Star City. This is underscored by her dying words.
- The season 7 flash-forwards feature Mia and William's struggle to form a sibling relationship after not being raised together. Mia feels abandoned by her father and betrayed by her mother. William is hurt that neither Felicity nor Oliver ever reached out to him after he went to live with his maternal grandparents. They initially both resent each other for the relationship they believe the other had with their parents in the past and judge the other as ungrateful for the gripes they have against their parents, believing only their own hurt is valid.
- In Boy Meets World, anytime Shawn Hunter angsts over his family life, he always tells his brother Jack they are just "half-brothers", implying they can never be truly close.
- In The Fosters, Callie was raised all her life believing herself to be the older sister of her brother, Jude. However, she eventually finds out that she and Jude only share a mother, and she has another half-sibling, Sophia, on her father's side. While she insisted that Jude is her brother and Sophia is just her half-sister, both were understandably shaken up by the news.
- House had Black Gal on White Guy Drama where the guy's father forbade him from seeing her. In fact, it had nothing to do with race; he had actually fathered both of them.
- Luke Cage: Willis Stryker is Luke's older half-brother, fathered by an extra-marital affair between Reverend Lucas and his secretary, while his wife had trouble conceiving. When Carl (Luke's birth name) was finally conceived, Reverend Lucas lost all interest in his first son, to the point of forbidding him from using Lucas as his last name or revealing is relation to Carl. Carl grew up Innocently Insensitive about the subject, often telling his "best friend" that it was up to him to protect the Lucas name. This, combined with his mother being discarded by Reverend Lucas and later dying alone, led Willis to resent Carl, eventually frame him for murder, and arrange for him to be tormented and experimented on in prison. These actions are, ironically, what led to the creation of Luke Cage.
- One Tree Hill centers around half-brothers Lucas and Nathan Scott. They were raised apart and found out they were siblings as young children playing in the same junior basketball league. It is a point of contention as Lucas is a few months older, but their father, Dan, chose to marry Nathan's mother, Deb, and lives with Nathan and Deb as a family. They all live in, Tree Hill, a relatively small town where the fact that Dan got two women pregnant at the same time is quite a scandal. Lucas struggles with the fact that he and his mother, Karen, struggle to get by while Dan supports a family in lavish style on the other side of town. Nathan struggles with the fact that everybody knows he has a bastard older brother who may actually be the more talented Scott brother but simply lacks the financial resources to compete at the level Nathan does.
- Tess Mercer (Lutessa Luthor) is the illegitimate, younger, half-sister of Lex Luthor. She grew up completely loyal to the Luthor name, wanting to further the family legacy to prove that she deserved the Luthor name. However, after finding out that Lex had an optical camera implanted in her brain in order to monitor her movements, Tess feels deeply betrayed and completely reverses her stance, spending the rest of her life helping Clark and thwarting Lex's attempts to find out the truth about Clark and his powers.
- Lucas Luthor is the illegitimate, younger, half-brother of Lex Luthor. After Lionel forced Lucas' mother to give him up for adoption, Lucas grew up in abject poverty When Lex contacts him hoping to undermine Lionel's control of Luthor Corp, Lucas jumps at the chance, but he is just as eager when Lionel pays him to betray Lex. Eventually, Lucas realizes that he's just a pawn in the game between his father and the only legitimate heir, Lex. Lex helps Lucas comfortably resettle where Lionel will never be able to find him.
- In season 3 of Sons of Anarchy it is revealed that John Teller had a love child with a woman connected to the Irish arm of the club. When Jax and Trinity meet, they are attracted to one another. They nearly have sex but are interrupted by their mothers who have to tell them the truth in order to kill the budding relationship. Jax is slightly surprised but not particularly upset. Trinity, however, is very upset as she was never told John was her true father or that she had siblings in another country, one of whom has since died before they could ever meet. She is very embarrassed the next time she has to face Jax.
- In season 5 of Supernatural, Sam and Dean find out they have a half brother Adam. Dean is very hurt to find out that Adam lived a normal life with John periodically stopping by for holidays (meaning Sam and Dean spent those same holidays alone in a run-down motel) and taking Adam for outings such as baseball games. The angst is mitigated when they find out the Adam they've met is actually a ghoul who murdered the real Adam and stole his memories as a trap to lure in John who'd killed one of the ghoul's parents (poor planning since John had been dead for years at this point).
- The Bible: The Book of Genesis combines this trope with Parental Favoritism among Jacob's sons. Despite having two wives and two concubines who give him twelve sons and a daughter between them, Jacob only really loves his younger wife Rachel. Therefore he favors her two sons, Joseph and Benjamin, especially Joseph. Unsurprisingly, his sons by the other mothers feel excluded and jealous, which finally leads to their selling Joseph into slavery.
- Averted in Fire Emblem: Genealogy of the Holy War and its midquel Fire Emblem: Thracia 776, at least by the ones who know they're half-siblings. Arvis loves his younger bastard brother Azelle, raising him from a young age, although Azelle is scared of him and Arvis orders Azelle's death alongside the rest of Sigurd's army as part of Arvis's coup. Lachesis worships her elder brother Eldigan, almost to BrotherSister Incest subtext. It is implied that Lachesis's two children Diarmuid and Nanna have different fathers (Diarmuid's father is Beowulf and Nanna's father is Finn), but they are ecstatic to be reunited.
- Arvis hypnotized Deirdre into marrying him without knowing they were half-siblings. It's implied that Arvis found out later, but continued to love Deirdre as his wife instead of his sister. Seliph and Julia, who share Deirdre as a mother, fall in love at first sight; whether they see each other as siblings or lovers depends on the adaptation.
- From Little Busters! is Haruka and Kanata, who are twins with different biological fathers. The problem is that Shou, one of the fathers, ended up imprisoned when the two were young, so the family was worried about raising the daughter of a criminal, especially since they didn't know which of the two was his daughter. Because they didn't know who was Shou's daughter, they made the two compete against each other to see who was the better one, with the loser being deemed a failure. This resulted in Haruka being deemed the failure, and being treated like the Black Sheep of the family. That being said, Kanata didn't have it any easier. Despite being deemed the family's successor, there was still paranoia among the family that she might be Shou's daughter. As such, whenever she didn't meet the family's high standards, she was beaten with a belt, and by the time of the main story, has been beaten so much that her arms are covered in scars.