I could see it happen between Anne and Maid, but stepping into sis and maid is fine too.
Just picture it, you hear a weird noise at night, and you walk slowly towards the source. You find your sister and the maid going at it, almost savagely.
Then, you have the choice to try to jump in, or keep watching.
Unlockable after completing maid and sis routes.
yeah, I can see it happen. Expect a sketch later.