The Succubus Sister Ending is an Ending in Prisoners of the Elder One. It occurs after asking Aubrey to use her transformative powers to restore Tiffany after giving her the Drugged Cake.
Taking some liberties, Aubrey transforms Tiffany to give her a dick. Before she could understand what's going on, the transformation continues, turning her into a demon and making Audrey both her big sister and her Mistress. As per the agreement to turn Tiffany back at the cost of his soul, Erik, Tiffany and Audrey travels to hell, where Audrey modifies Erik to be her perfect sissy lover, and Tiffany quickly becoming her most influential underling. Though the two siblings are allowed to meet whenever Audrey is bored, their mental transformations mean that they gradually drift apart and unable to recognize one another.