Nanase is going to make it up to Yuni. Yuni will forgive Nanase because she loves her and she'll break things off with Fuuko. Yuni will be neglected by Nanase again and Fuuko will undoubtedly use the opportunity to cement her side piece status.
The question is will the cycle break? Will Yuni get sick of the back and forth with Nanase and choose Fuuko? Unlikely but you never know. Will Nanase discover and break things off? I feel like she'll find out but maybe be okay with sharing? I mean, she gets to focus on volleyball and doesn't have to worry about fulfilling girlfriend duties all the time since someone else is picking up her slack. Basically sex without responsibilities... And I'm pretty sure Fuuko can mindf*ck Nanase into feeling guilty and blaming herself.
Fuuko and Yuni are two mistresses that are hanging on in hopes of being updated to wife status. They are perfect for each other. Granted we don't know much about them but, from a logical standpoint, they'd fit each other better. They'd understand the other more and certainly would have time to devote to one another.