Most people know who they are related to. Most people know their friends' family, too. However, in fiction there are many ways to create story tension by having a relative be unknown before leading into a reveal.
The work has Alice, who has been friends with Bob for a long time. In season 4, Bob's sister Claire comes to visit and Alice does not know who she is, despite it being a reasonable expectation for her to be acquainted with Claire after knowing her brother so long. Bob may be on vacation and doesn't introduce them, leading to awkward hijinks occurring before it is revealed who Claire is to both the characters and audience. Perhaps Alice has been dismissive of Claire, or mentioned how much she feels threatened by her friend's sisters.
This is not the only circumstance, but is a common one. Another common variation is the character themselves not knowing of a relative of their own, that person then coming to meet them — in these cases it is more likely that the audience already knows.
A character may also be hiding their relationship to a family member from their friends for a good reason, though that family member may be frequently present.
May be foreshadowed by having the same surname. The interactions may go on for a significant length of time before the reveal, sometimes with everyone but the relative not knowing, though usually the identity will gradually be told to more people.
See also Remember the New Guy?, when a never-before-seen character is treated as if they've been through the story with everyone else (making them an unknown relative to the audience, rather than the characters).
Super Trope to Long-Lost Relative, for specific cases of when both parties do not know they are related, Your Son All Along, when the relationship is a father-son one that the father doesn't know about and Luke, I Am Your Father for the other way around, and Relative Error, when someone doesn't know that two people are siblings and thinks they are dating.
This is not for examples of when two people have alternate identities and don't know that their masked partner/villain is related to them.
The unknown relative may be played by an actor who's a relative unknown, but you may be looking for Darkhorse Casting.
WARNING: As a Reveal Trope, spoilers will be unmarked.
- One Piece: Luffy apparently never even thought of having a father (he only remembered having a grandpa) until his grandpa Garp points it out. It, of course, enables a reveal: that Luffy's father is an infamous criminal (the most wanted man in the world, in fact) named Dragon. It is implied that Dragon never raised Luffy and just left him in his hometown in East Blue.
- Flame of Recca: Early in the series, Recca has an antagonistic relationship with Kage Hoshi due to pitting his friends against him. However, he finds out later that she is his mother, Kagerou, who actually saves him from their village massacre 400 years ago by teleporting him into the modern age which results to her becoming immortal. Her actions in the first arc is a test to see if her son is ready to fight stronger enemies.
- In Danny Phantom fanfic ResurrectedMemories: As stated elsewhere the little girl Ember saves from Skulker and her big sister whose band Ember ends up joining turn out to be Embers nieces, a fact the three of them are completely oblivious to at the moment.
- RWBY: Scars: In Volume 5, Ruby meets her maternal aunt Ivory while traveling. Ruby's mother Summer had never even mentioned having an aunt.
- After the Wedding: Jacob, a Danish aid worker who is getting a donation from a rich philanthropist. He gets invited to the rich guy's daughter's wedding. He's pretty startled when he shows up at the wedding and sees that the bride's mom is his girlfriend from 20 years ago. It doesn't take long for him to realize that the bride is actually his daughter.
- In Wanted, Wesley doesn't realize that Cross is his father until after he kills him. This is foreshadowed by the fact that Cross seems bent on killing everyone but Wesley.
- House of Anubis:
- Though Patricia had been in Anubis House for what's implied to be several years, only her best friend Joy seemed to be aware of the existence of her Polar Opposite Twin, Piper. This allowed for a subplot where Piper stayed at Anubis House in secret, inadvertently tricking people like Alfie into thinking Patricia was suddenly being nice (and thus into thinking the crush he had was on her rather than on Piper). The moment when she revealed to everyone in the house that Piper existed was full of shocked reactions that could've only happened had Patricia simply never mentioned Piper's existence. It's implied her jealousy of Piper's talents and social skills were why she kept everyone but Joy in the dark.
- Season two introduced Jerome's younger sister, Poppy. While he'd been secretive about his poor relationship with his parents until he opened up to Mara in the first season, he never mentioned having a little sister until she appeared at the school. Poppy herself exploited this, as she used Jerome's unwillingness to admit her existence to his friends as a way to blackmail him at the beginning of the season.
- Grey's Anatomy:
- Arizona picks up a girl at Joe's and takes her home, and it looks like it could be a successful one night stand for her to get over Eliza and finally uncomplicate her love life... until her long-time roommate and good friend, Andrew, comes home and scurries past — but the girl notices him and calls his name. She reveals that he is her little brother, something you would imagine Andrew would have probably mentioned to Arizona (since they have lived together for a few years and he testified for her at her daughter's custody hearing), or if not to have at least had a photo around that Arizona would have recognized.
- Meredith started the show as an only child, discovering over many seasons that she has at least three half-siblings that her parents didn't even hide from her (Molly and Lexie from her dad, Maggie from her mom). She visited her dad a few times before meeting his daughters, apparently just never bumping into them or seeing photos, and was there for Maggie's birth but... forgot? So that Maggie could dramatically reveal it when she gets hired as Cristina's replacement and give Meredith the realization that Maggie's father is Richard Webber, which neither of them know. It's surprising that Meredith's mom's Alzheimer's didn't have her ever mention it.
- In the Star Trek: The Original Series episode "Journey to Babel" Kirk is surprised to meet Spock's parents. It's a little unrealistic that a Starfleet captain tasked with transporting a distinguished delegation to a vital conference would have no idea that Vulcan's ambassador to the Federation is his first officer's father.
- Zero Escape:
- In Nine Hours, Nine Persons, Nine Doors, Junpei was childhood friends with Akane but doesn't recognize Akane's brother Aoi.
- In Virtue's Last Reward and Zero Time Dilemma, Sigma (father), Diana (mother), and Phi (daughter) are all unaware of their familial relationship. (Because of time travel shenanigans, ZTD is set roughly ten months before Phi's birth. Do the math.)
- Voltron: Legendary Defender:
- Lotor's backstory involves a scene where, as a child, he questions his father about the identity of his mother, not realizing said mother was in the same room and in his line of sight. It's implied that as an adult he did his own research and discovered Honerva had become Haggar, but refused to accept or acknowledge it.
- Keith spent his childhood knowing nothing about his mother aside from the fact that she had left when he was an infant and was once in possession of his Orphan's Plot Trinket. It's not until his late teens that he learns that she is a Galra woman named Krolia who works with the Blade of Marmora, and her apparent abandonment was a case of Give Him a Normal Life.