Replacement Sibling

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.

Anime & Manga

 * Minagi in AIR
 * Naoi Ayato of Angel Beats had a 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.
 * In Fushigi Yuugi Nuriko takes on the role of his dead sister.
 * Ken from Digimon Adventure 02. After his brother Sam dies, Ken starts trying to be more like him to please his parents.
 * 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). Rape Is Ok When It Is 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.

Film

 * David Keith's character in An Officer and A Gentleman doesn't want to be a Navy pilot, but feels obligated to follow in his late brother's footsteps.
 * Used in the slasher film Sleepaway Camp.

Literature

 * My Sweet Audrina appears to be this at first, but it turns out
 * 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 assasinated onboard Imperial Terra, Colin and Jiltanith need to also secure the succession of the empire by having more children.
 * The Olsons on Little House On the Prairie adopted Nancy after Nellie left Walnut Grove.
 * The book North Child (also known as East), where Rose is born to replace her elder sister.

Television

 * 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.

Web Comics

 * In Drow Tales when this happens, the replacement child is often renamed.

Real Life

 * Allegedly: JFK. (His older brother was killed in an accident.)
 * Truth in Television. Some cultures even believe that a new child born shortly after the death of an older one is the child, or at least God's way of saying: "Sorry I had to take your kid away. Have a Replacement Goldfish!"