I was here not too long ago. I think you need to do the Night Interactions with Christa. Like I mean go for her pussy or something and then get on top of her. When you do that Ymir automatically catches you doing it.
but that's the problem when i try to do any off the night interactions they just give me a bit of dialogue and then end without doing anything i have try taking the black off but nothing same with the pussy and ass.