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, interact with Ixies on the bed and exhaust all dialogue options, 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. Once all of this is complete, leave via Golden Gate. This results in Ixies secretly coming to Earth in the guise of Tiffany and seducing Erik. Meanwhile, the real Tiffany has been turned into a mindless sexbot and awaits further orders.