Sorry, I thought you're on Charlie's path
Where did Charlie confess about her feelings to Lucy?
On Lucy's path she says that she has no romantic feeling towards him. Sorry I was thinking about another scene further in the game.
I don't remember what happened before the bath scene.
However, I can agree about the absurdity about the choices. If you choose one sister, then the other one automatically stops developing anything more than friendly feelings towards the MC without any conversation between her and the MC or her sister. The sisters seem to be telepathically linked and know about the choices you make about the other sister
Consider it a comical plot hole.