this ending is activated after you free Ixies from the robo-golem that trap her.
you must spill cup of water on golem to free her. after that you will not be able to get strange crystal from the books next to the bed.
when she is on the bed, tell her ,,You have a lot of stuff´´she will ask you if you want to look at hentai,chose ,,sure´´. interact whit stuff in her room(all of it) she will interact whit some objects if presented. if you have sword whip use it on her (the sword whip can be found in the cells south of necromancer, on a table) after that free your sister and change her back to human and exit thru golden door.
if every step was done at home your sister will say ,,how could i sleep badly whit you here to protect me,brother?´´ and that is it folks!