Two Siblings in One

"Hey, Rubedo! Be sure to breathe deep and inhale the particles that composed me!"

—Albedo, Xenosaga II.

This occurs when a character's sibling dies and the character absorbs their soul, or the dead may force their Life Energy on to their sibling. The rule of "two souls cannot control a body at the same time" usually applies and the siblings will switch control over the body. The original owner of the body is usually in charge, but the dead sibling will take control at key moments. Usually accompanied by friends of the 2-souled-twin saying he/she is Not Himself. In most cases when the inhabiting sibling is in control, a distinct feature will let the audience know. This can range from Mind Control Eyes, to a colored aura that appears around the character, in some cases the voice will also change.

Compare Body and Host and Sharing a Body. Literal version of My Sibling Will Live Through Me.

Examples of Two Siblings in One include:

Anime and Manga

In Le Chevalier d'Eon Lia forces her soul on to D'Eon so that she can take revenge on her murderers.
In SHUFFLE! Sia absorbed her twin sister Kikyou's soul.
- Nerine has a similar thing with Lycoris, although in this case it's "clone" rather than "sibling".
In Chobits Freya inhabits her sister Chi's body, sometimes taking control, but otherwise remaining dormant.
Invoked by Hansel and Gretel in Black Lagoon. They're siblings, but who's the boy and who's the girl? Good question. They change around at will - and when one of 'em winds up dead, the survivor happily declares that they're still together, even as she changed into a he... (Note that all we see is a change of wig and voice - but it is implied that whatever physiological difference there may have been between them, is no longer visible without an autopsy.)
Sakon and Ukon from Naruto.
Hanako and The Terror of Allegory: Dead sibling Nozomi takes over Kaede's body as part of a revenge scheme.
Psychic Academy had a pair like this, where the sister has her brother's heart. Comes as a surprise, because in the early part of the story it is the brother that is dominant (and The Antagonist).
In Basilisk the Big Bad of the group apparently absorbed his twin brother. His brother is still alive, so to speak, as a monstrous symbiote who gives his brother immense regeneration in return for surviving freakishly. This also counts as squick, since said symbiote is only able to speak through his brother's ripped throat.
This is the setup with Yayoi and Uzuki in Softenni. It helps that Yayoi comes from a family of spiritualists.
In Persona Trinity Soul, the main character's brother had a twin sister that died when she was young. Her brain was transplanted into his. While he remains in control for the duration of the show, at the end of one episode, he turns into her.
In Inazuma Eleven, after Fubuki Atsuya dies in an avalanche, his brother Fubuki Shirou develops Multiple Personality Disorder, with the second personality being Atsuya's.
In Darker than Black Hei's Contractor Power comes from his sister, who joined with him during the Heavens Gate Incident.

Comic Books

This was the origin of Golden Age DC Comics superhero Captain Triumph. When Lance Gallant swore revenge for his brother Michael's death, Michael's ghost appeared to him, and by merging they gained superpowers.
Bronze Age Marvel Comics character 3-D Man was the soul(?) of a dead brother, brought back to life by the live brother when concentrating through his glasses.
Marvel Comics' Brother Voodoo with Jericho (living) and Daniel (spirit) Drumm. Daniel could leave Jericho's body to possess other beings.
Haunt is about a Badass Priest haunted by his brother, a deceased secret agent, becoming a superhero.
Firestorm from DC Comics was this, before Doing In the Wizard and becoming a fire elemental.

Fan Works

In the Ranma ½ fic series The Tales of Ranma and Ranko by Jack Staik and Lady Tesser, a magic mirror splits Ranma and Ranko, the spirit of his girl form, into two separate individuals. Some time afterward, they learn from Ranma's mother Nodoka that Ranma had a stillborn twin sister, and Ranko believes that she is/was that sister.

Literature

In the Disgaea Novels the twins Shas and Kira came to share Shas' body when Kira died shortly after being born.
The plot of Haunted Sister by Lael Littke. Janine's twin Lenore died when they were young. After a near-death experience where Janine goes to heaven but is told it's not her time, Lenore's soul comes back with her. The two don't get along very well, and Lenore increasingly tries to take over more.
Thad Beaumont/George Stark from Stephen King's The Dark Half. An odd example in that it starts with Thad put his alter ego to rest. But then Stark gets a body of his own...

Live Action TV

In Heroes, Niki Sanders manifests the consciousness of her dead, older sister, Jessica Sanders, and gains super strength when she does so. Jessica died protecting her sister from their abusive father and continues to protect her, even if she uses violent and sociopathic means to do so. It was originally not clear whether this was actually two minds in the same body (though "Jessica" seems to think so) or just regular old Multiple Personality Disorder.
- It seems to be "normal" crazy, considering Niki was eventually able to use the powers "herself", and that she eventually manifested other personalities.
The Outer Limits episode "Inner Child" explores this when a woman is attacked, wakes up in the hospital, and finds out that she had a twin that died and was absorbed into her body. The twin starts taking over (with the eye color changing to indicate who is in charge), but ends with the twin sacrificing herself and giving control back to the living sibling.

Video Games

In SaGa Frontier, Blue's quest involves gathering magical knowledge and killing his brother, Rouge in a Duel to the Death, to attain his true strength. Obvious spoilers are obvious, given the trope we're dealing on. In game, there are several opposing types of magic (such as light/dark and time/space, and the player can only obtain one of each set. Whatever Blue doesn't obtain, Rouge does. The winner of the battle to the death absorbs the loser's power and thus obtains every magic in the game.