There's literally a segment where the MC has an emotional breakdown and instead of caring for him while he enters a delusional state his mom and sister have sex all over the house with their partners. If that ain't peak "You don't matter to me anymore" behavior then I don't know what is.
I don't think CotV is going to go that far simply due to how it's set up. I could, however, see Ryuu getting drunk and missing out on his mom and sister getting fucked. I just have a hard time visualizing Marin sneaking someone like Enlai into her room.
Although I do believe that the time will come where Enlai enters Marin's room, I agree with you, inviting someone of Enlai's appearance to your house or your room is not easy to explain if someone from your family sees you haha.
Maybe solution could be the tree house, if I remember correctly that place is very important to Ryu, so it could be a sign of betrayal or Marin's corruption.
Although I think using that place is also difficult, I don't know if Inatari already has a plan for that place, or it will become one of those places where nothing happens until the end of the game.
But one option could be that Enlai loses his store (maybe he himself is the one who destroys it hahaha) and Marin being a good person and because of corruption begin to feel a little empathy for Enlai invite him to stay in the tree house while he recovers his home, although Ryu and his father are not very convinced, they decide to accept because Enlai was left homeless, and as it will only be temporary they don't see any problem and Violet, well, she is more worried that they don't discover her relationship with the grandfather that she doesn't mind letting Enlai stay in her son's tree house
It is a difficult scenario, I know, but I think it can be a solution for Enlai to enter Ryu's house without problems since he will have permission to enter only to use the kitchen and the bathroom.