The A New Sister Ending is an ending in Prisoners of the Elder One. It occurs after leaving through the golden doors without saving Tiffany, and agreeing to take her place.
The Elder One was not being entirely truthful in his promise, for when Erik wakes up, he sees that he has become Tiffany; the real Tiffany is still a slave to the mansion. Not only does 'Tiffany' share her sister's appearance, but also has some of her mannerisms, though her identity is not wholly replaced. This means that on some days (such as when she's with her boyfriend), she is but a prisoner in her own body.