Hina obviously loves Rei and of course her sister too. When you really love someone you just wish all of the best for them, and for Hina, that means them getting together. As long as Rei can stay by their side, Hina is probably going to be ok with anything, if even that means suppressing her own feelings. Her logic is most likely to be: if people I love are happy, I'll be happy. By the way, I think the crush she had on the baseball guy was probably more admiration than love. I think Akari mentions this point in one of the previous chapters when she was discussing with their aunt about Hina's love interests. In my opinion, Takahashi just left too soon for her to develop real feelings.
The thing is that Hina doesn't realize she's the key to Rei's happiness. Yes, Akari physically saved him, but Hina also saved him, psychologically. All the Kawamotos have been extremely helpful, but it was Hina who really changed him. Moreover, I don't think Akari has any romantic feeling towards him(?) Well, it's more like she doesn't think about this kind of things. And she knows their feelings towards each other so...