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I've tried several versions of this story over the last year or so; but this one feels better than all of them. It's called My Sister The Alien - and it revolves around 18 year old Olivia Shepard. Her (older) twin sister Deanna had been diagnosed with an aggresive cancer when they were 3 years old. Some new neighbors took an interest in Olivia and Deanna; once Deanna's condition worsened they revealed the truth - that they're alien scientists studying Humans.
They offer to cure Deanna; even if the cure means that she'll become a member of their species and have to leave Earth. The parents agree to this.
12 years later an accident on a hike leads to a reunion being Olivia and Deanna (now known as D'ana) on the latter's adopted homeworld. The nature of the accident means that the treatment will turn Olivia into a member of the same species.