Sometimes when a parent loses a child, they create or adopt a new one to take the dead one's place. But why waste the time and effort on that if there's an existing sibling to fill the role? Sure, it won't be a perfect fit, as the other kid has had at least a few years to develop an identity of their own, but they probably look kind of like their sibling (even if they're of the opposite gender), and they're likely to go along with it just to keep their parent(s) happy. Or maybe they don't have a choice in the matter because the parent's just crazy enough that they can't be convinced that the living child isn't the dead one. Though sometimes the child will play along only at home, and continue to be himself at school or work or what have you.
If the living sibling takes on the role of the dead one willingly, it's My Sibling Will Live Through Me.
Sometimes this happens when the first child is still alive, but has gone away.
- Ayato Naoi of Angel Beats! had an older twin brother who was a genius at the family trade, pottery, while Naoi's own skills were pretty average, making Naoi decidedly The Unfavorite. When the brother dies, their father forces Naoi to pretend to be him.
- Ken from Digimon Adventure 02. After his brother Osamu dies in an accident, Ken starts trying to be more like him to please his parents. Thanks to the Seed of Darkness inside him, he instantly became a Child Prodigy like his brother and the parents treated him like they treated him Osamu. It isn't until Ken returns from the Digital World after three months that his parents realize the mistake they did and they start to love him as he is, not as a replacement.
- Hali Kang in INVU
- This occurs in Secret Plot Deep, where a boy's parents took his twin sister's death so hard that they've become convinced that he was the one who died instead, forcing him to impersonate his dead sister at home (and change into a girl's school uniform before leaving school). Double Standard: Rape, Female on Male ensues, oddly enough.
- Michel in Glass Fleet combines it with Sweet Polly Oliver - she's the real Michel's younger sister, taking his place following his death in order to continue his cause.
- Ringo Oginome's character arc in Mawaru-Penguindrum is based around this, and her complex over her Dead Older Sister Momoka, who died the day she was born. Notably, Ringo's parents didn't want Ringo to do this, but she became convinced that it was the only way to keep their family together.
- Toyed with in Ten Yorimo Hoshi Yorimo. The Narumiya family adopted a girl named Rei to fill in the void after the death of their son Shou in a fire... but in reality, Rei is Shou himself, who survived the incident but had to go as a girl for years to avert the persons who wanted him dead.
- In Kore Wa Koi No Hanashi, Haruka's mother began to pretend that Haruka is her deceased brother Kazuma. It doesn't end well when Haruka doesn't "play along".
- Cheshire, back when she was still a member of Secret Six, conceived a child with Catman for the sole purpose of replacing her daughter Lian Harper, as she was planning to betray the team but was being threatened by the team's benefactor with the death of her daughter Lian.
- My Sweet Audrina appears to be this at first, but it turns out the titular character is the First Audrina, who was raped and underwent electroshock therapy in order to erase the event from her memories. After that, her parents made up the story about the First Audrina.
- Dealt with in several of David Weber's works including Honor Harrington where her parents believe her dead and conceive to amongst other things secure the succession of Honor's Steading, and in the Empire series after Sean and Harriet are believed to be assassinated onboard Imperial Terra, Colin and Jiltanith need to also secure the succession of the empire by having more children.
- The book North Child (also known as East), where Rose is born to replace her elder sister.
- Flora Segunda in Ysabel Wilce's Flora trilogy.
- Paige, the half sister of the Halliwell sisters, replace Prue after her death.
- An episode of the HBO series In Treatment has an older man revealing that when he was a child his parents decided that he should replace his older brother after the latter died in an accident.
- This is how Kenny is brought back from the dead as seen in one episode.
- In My Family after Abi moves out, Susan tries to make Michael the new daughter. Much to his annoyance.
- In La Femme Nikita, when Seymour Birkoff dies, he is soon replaced by his twin brother Jason, with a completely different personality. Yes, we had met Jason before, so it's less of an Ass Pull than it sounds.
- The Olsons on Little House on the Prairie adopted Nancy after Nellie left Walnut Grove.
- On Downton Abbey cousin Rose comes to live with the Crawley family after Lady Sybil dies. While she isn't officially adopted, Rose more or less becomes another daughter, replacing Sybil as the youngest sibling.
- In the Law & Order: Special Victims Unit episode "Locum", the detectives investigate a couple who are forcing their foster daughter to serve as a replacement for their long-missing biological daughter.
- Minagi in AIR takes on the role of her stillborn sister Michiru, to the point that her own mother forgets Minagi even existed and calls her Michiru.