I get the impression some of the early posters in this thread didn't grasp what the situation is (and has been) between Umi and Yuu. Yuu likes Umi (as a friend) a lot, and would do anything for her. Unfortunately, Yuu has a +900 charisma stat, and Umi... doesn't. Their mutual friends (including boys Umi has been interested in) tend to get bored of Umi and start trying to do things only with Yuu, leaving Umi out. It's happened over and over again over the last few years. Essentially, Yuu has (unintentionally and through no fault of her own, other than failing to notice it happening) stolen everything Umi has ever cared about. Umi, throughout this releationship with Maki, has been afraid for Maki and Yuu to meet, because she feels like Yuu will steal him, too. Umi does value her friendship with Yuu, and she knows that it's the other people around them doing it, and not Yuu herself, but she still resents Yuu because of that. This is what led to her keeping her relationship with Maki a secret to begin with.
Yuu trying to force the two of them together in this chapter blew me away. Based on the previous couple chapters I was convinced that she was actually falling for Maki as well (which had us well on the road to repeating what Umi feared most, if Maki reciprocated). If Yuu really did fall for him, I can't believe she'd back off like this, at least not so quickly. There's some degree of "I value my relationship with Umi more than I want a new boyfriend" I suppose. But maybe her doing this in this chapter will make her realize what she's throwing away. And personally, I hope Maki sticks with Umi regardless.