Fuuko definitely wants to be Yuni's ride or die and I'm here for it. I may have been correct in insinuating the mom play was a way to ensure that Yuni wouldn't be able to reject her advances. She's posed herself as a caring female figure that Yuni would have trouble saying no to. I mean you can argue with your sister but your mother, not so much. Plus, it feeds her need to care for Yuni. She wants to be Yuni's always and is willing to fight, underhandedly I know, to keep her position in Yuni's life. A fact I think Yuni is starting to appreciate but can't bring herself to admit. People that are saying she hasn't learned anything, I think that's not true. She's learning to assert herself which, from what I've seen with her family life, is growth. Hopefully this'll transfer over to the other aspects of her life and she can tell her awful family to f#ck off.
I think the problem isn't that Yuni is selfish but she has abandonment issues so she can't bring herself to kick out someone that wants to be in her life, which Fuuko's using to her advantage and because of how affectionate and caring she's been it's working to tip Yuni's favor her way. Yuni wants Nanase to leave, can feel how incompatible they are and see how they're repeating the same patterns, but doesn't have the strength to cast her out, especially not when she thinks Nanase loves her. By now she knows she's made a mistake in choosing Nanase but she isn't strong enough to turn her away. She wants Nanase to make that choice.
If Nanase truly wanted to, she could have grabbed Yuni and taken her to the infirmary herself... at this point I'm thinking is it her competitive nature as well that's keeping her bound to Yuni? Yuni was essentially her doormat and when she found out someone else was interested she promised to up her game and Yuni chose her. Then they tried and are currently failing at connecting. But I feel Nanase doesn't want to see it? Perhaps because she's goal oriented and right now her goal is to keep Yuni with her and rub that she's the victor in Fuuko's face... now that it's apparent that she isn't "winning" she's becoming more volatile because she's not so sure how to handle yet another loss of a relationship since her other attempt at a relationship was botched.