The Fake Sister Ending is an ending in Prisoners of the Elder One. This ending can be achieved by freeing Ixies from the robo-golem that trapped her. To do this, you must spill water from the Plastic Vase on the golem, which causes it to short-circuit and explode.
Afterwards, when on the bed, interact with Ixies and exhaust all dialogues including reading erotica with her and showing her the Sword Whip. Also interact with the various objects in her room until she decides to talk with Erik about science. Finally, ensure that Tiffany is fully restored and leave via Golden Gate. This results in Ixies secretly coming to Earth in the guide of Tiffany and seducing Erik. The real Tiffany meanwhile has been turned into a sexbot and awaits orders that may come later.