Two Sides, Same Coin (джен)

Of all the things Heather Mason had expected to happen to her in her short, seventeen-year long life, running into a dark-haired doppelganger in the mall bathroom wasn't one of them. Especially when that same copy of herself was a constant presence in her nightmares while growing up.
Despite her fears of it carrying out some sort of unspeakable act, Heather quickly learns that her copy, who calls herself Alessa, was nothing more than a tormented individual who wanted to be free from it. This moved her enough to make an attempt to help her, which put Alessa on the path of becoming part of her family and by extension, her sister. From then on, her dark-haired doppelganger shared a fine enough existence living under the same roof as Heather and her father, Harry Mason.
However, that isn't to say there aren't moments where her new twin "sister" drives the young Miss Mason up a wall.