You know, I can actually imagine Shiemi/Shima easier than Izumo/Shiemi. If there's one person who would be able to forgive Shima, it's her.
Also he didn't spend the whole story creeping her out.
Haha, somehow, I think a lot would be easier than Izumo/Shiemi because of the whole same-sex thing. Unless you meant to say 'Shima' instead of 'Shiemi'?
Up to that point everything showed Shiemi was in love with Yukio and completely oblivious to Rin. Then after being kind of rejected by Yukio, Paku makes her realize Rin loves her. The next day at the festival, she hesitates so Rin tells her "I get it" before going to get Yukio. Then she think to Rin "I like you".
So either she still loves Yukio or she doesn't know who she loves anymore. But she likes Rin for understanding/respecting that.
If she doesn't love Yukio anymore and now loves Rin, why bringing Yukio would make her happy? That would be the guy who just rejected her and the guy she just decided she doesn't love anymore. That doesn't make any sense. If she just decided she loves Rin, she'd tell him, and she would want to be more or less alone with him, at least not along with the guy she just decided she doesn't love anymore.
My cell phone is dying and I got to go.
Maybe I didn't think this through enough, but that's what I saw while reading. I'd like to call the shiping-goggle on people who can't see that, but I'll be fair so I won't.
It's quite a bit more complicated than that. Yukio wasn't 'rejecting' her at that point - he had the role of a teacher at that dance, regardless of his age, so of course he couldn't dance with one of his students. Whatever he may or may not feel about her, that's irrelevant.
I think Rin was just trying to make the best of matters and have the three of them getting along. Which I feel is what Shiemi wants, too: no matter who ends up with whom, I think both her and Rin would like to be comfortable with each other and with Yukio. (Yukio's nii-san complex and mess of feelings are another matter.)
I also think that, because Shiemi is very new to the idea of social interaction, she doesn't always distinguish between friendship and romance. To be fair to her, it's not always easy for other people to do so, either. It's obvious that she's trying to discover what her own feelings are. Complicating it for her is that she couldn't even see Rin had feelings for her until Paku pointed it out herself.
Replying to your last paragraph, A) it's possible to be in love with two people at once, B) different people handle perceived rejection differently, and C) I still say Shiemi wants the three of them to get along, regardless of who's together with who. Even if Shiemi had felt "rejected" by Yukio (which she wasn't because again, teachers are generally not allowed to dance with students at these events and Yukio had to make that clear), it doesn't necessarily mean she'd want nothing to do with him. I also don't think she's firmly decided that she's "in love" with Rin. People don't fall in and out of love that easily. I got the sense that she meant she liked him as more than just a simple friend, though not enough to want to be in an exclusive relationship with him. Does that make sense? Her feelings are muddled, and right now, she just wants to enjoy the dance.
tl;dr romance is complicated and illogical and looking for sense in it is like looking for water on the moon.
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