Effortpost incoming, I just really love this shit.
Fuuko is, in my opinion, the most interesting character in this manga. She has abandonment issues similar to Yuni but not quite as severe. It seems that Fuuko's parents were at least present in her childhood, whereas Yuni was left to fend for herself more or less while her mom worked; she had no siblings and no dad, leading to her intense attachment to her cat.
Fuuko's core issues stem from the complete lack of affection and (positive) attention from her parents - they only had anything to say to her when she failed to live up to their impossibly high expectations. Constantly being compared to her siblings instead of valued for who she is as an individual, combined with the romantic involvement with her neighbor (hint: re-read volume 1 and look at the hairpieces the two of them are wearing. The moon and star motif didn't come from nowhere...) has led to her having very high walls to protect herself from abandonment and rejection. In Osaka, Yuni notes how different Fuuko is in class versus when it's just the two of them. In contrast to her aloof public persona, the true Fuuko is hurt and still unable to move past the trauma of her childhood. Though she makes these grand overtures of love and affection to Yuni, she does not truly understand these feelings because she was not able to work through them - she was left holding the bag when her neighbor left her. This was also the one person in her life that showed her real affection and cared about her feelings, who she is as a person, and losing her abruptly would have felt like an unprecedented betrayal - "why did you leave me? You knew who I was and you loved me - why was I not enough for you? What did I do wrong? Why wouldn't you put me first? Why won't ANYONE in my life make me a priority, just once?"
This is critical to understanding why she wants Yuni so badly. She sees parallels to her past relationship in Yuni's one-sided relationship with Nanami. In the very first chapter, she says "I thought maybe you would understand what it feels like" - I don't think she was JUST talking about being attracted to girls. She knows what it feels like to be a footnote in someone else's story, an afterthought, a relationship of convenience.
As for Yuni, look at what she does when she's not around Fuuko or Nanase. Her entire personality is being in a relationship. In every scene with her and Nanami, Nanami is the one doing things while Yuni is a passive companion, at most eating food or posing for a photo. This is what Fuuko wants - someone that will fawn over her, dote on her, never leave her, give her all of the attention and validation that she so desperately desires. She could not care less if Yuni has no ambitions or interests - that's perfectly fine with her as long as she stays and loves her.
Now to bring it back around to this absurd comment, "I want to be your mama". Fuuko is initially disgusted at the idea of a baby, probably because of who it came from, but when she holds it she can feel the love radiating from it. Pure, unconditional, unlimited love. She feels this viscerally and realizes in an instant that this is what she wants, and she wants it from Yuni. She is saying that so long as Yuni will give this affection, attention and love to her, that she will take care of her in every way. There is a more sinister side to this as well - a baby cannot really say no, and they cannot leave. They're a captive audience to their caregiver, for better or worse. Well, that's just perfect for someone with abandonment issues, isn't it?
Finally, why does Nanase stay? I think she's basically trauma bonded with Yuni. She is clearly not over her own tumultuous prior relationship, and is deeply insecure about herself. The very first scene we see of Nanase is her getting a new hairstyle and worrying about Yuni's opinion of it. Sure, it's an innocent and innocuous enough thing, but I feel that there's a reason that this was the very first thing we learn about Nanase and the very first interaction she has with Yuni. She cares a lot about what others think of her, way too much, and contorts herself to be everything for everyone at the cost of her most intimate and personal relationship, a classic symptom of someone with poor self image and an ill-defined sense of self worth.
The only hope for them is for one of them to have some self awareness and figure out what's really going on. Yuni has moments of clarity where she comes close to self-reflection but she is likely afraid of what this would mean for her. The chapter where Nanase takes her to tour college campuses - it was obviously uncomfortable for Yuni because of the presence of Nanase's friend, but that's not the only reason. She had a brief crisis because she became intensely aware of how empty she is, how her only interest is being around Nanase - she is defined by her current relationship and nothing else. That's a hard pill to swallow for anyone. Doubly unfortunately for her, that's precisely what Fuuko wants more than anything.