After reading the last few chapters again, I think the main issue is that they just aren't on the same level emotionally yet. Yuzu is struggling with her emotions. I think she does accept her feelings for Mei, but is having a hard time trying articulating them properly. Also I think while she would like to be intimate with Mei she is afraid. Not just of betraying her role as a sister, but also she doesn't want to feel like she is just using Mei to sate her desire. Since for Yuzu I feel she wants to connect with Mei and not just use her as an object like that teacher did. The problem is Mei is so guarded, aloof, and casually about intimacy it is just twisting Yuzu up inside. She seems so desperate to connect with Mei, but she panics and trips over her words when Mei doesn't react.
As for Mei since we are meant to follow Yuzu we are also in dark about a lot of what Mei's actions and words mean. Unlike a lot of people I don't see malice in Mei's action here. It's building towards something. Frankly I think Mei is just confused about how Yuzu feels. Mei was more than willing to offer herself up to Yuzu at Christmas, but was pushed away. I think Mei is having a hard time trying grasp that Yuzu needs more from her than just her body. I do think in her own way Mei was being kind. I think she is having a hard grasping that while Yuzu does desire her, it is more than just a physical attraction. I may be wrong but I think she has a hard time believing that someone could be that in love with her. Since all the other in her that have "loved" were either using her or gone about it in superficial way.
Now I could easily be proven wrong with the next chapter, but I think Sara's confession made Mei realize something. Which is why I think she had that slight smile on the two-page spread. Mei asked Sara if she needed her and I think Mei finally got it. Love is about depending on one another, caring for another person, and just basically trusting each other. i feel it all fell into place for her at that point.
As for the last page, I think Mei is playing at something here. I am not sure what. Maybe she feels she is broken and wants to spare Yuzu. Or she is testing Yuzu to see if this what she really wants. There is something going on since I doubt Mei would just fool around with Sara. It is going to be a long wait for the next chapter, but I think this volume will end up bearing fruit.
Excellent analysis, you're on the right track. Everything is building towards something.
Nina does know that Mei's interested on Yuzu and yet she kept pushing the issue. Why?
Mei is hoping something from her "relationship" with Sara
This events might be what Yuzu needs to finally become proactive reagrding Mei
Sara is up from some long overdue reality check regarding her beliefs and her "little Sis"