Agreeing with Anya here. As usual, Shiemi makes her decisions based on helping her friends - not out of romantic desire.
Even if it wasn't romantic, it does show that Shiemi doesn't care all that much about Rin. Or rather, she does care, but she's not really thinking about him. Last time in the Impure King arc, she was beating herself up because she hadn't noticed Rin's depression, but now she's doing the same thing again: not thinking about his feelings. It's not even that she's purposefully ignoring them, the question just never enters her mind. I mean, did she consider he might want to go to the party too? She must know he doesn't have many female friends. Did she even wonder what he wanted to ask her (you know, when he came running to her table all out of breath) at all?
I think she just doesn't notice romantic shit. Hell, she doesn't even notice when someone is insulting her.
Shiemi only invited Yukio for one reason - because Yukio has been acting tremendously out of character. She notices this because she's been around Yukio for several years. She wanted to use the concert to make him feel better. Nothing romantic about that.
She's only known Rin for half a year. She's never dealt with any other guys. Of course she doesn't notice the signs that he likes her.
Once she does know about it, she'd probably feel very upset. But up to that point, she's simply oblivious and is imposing her skewed views on the situation. I can't blame her for that since none of the other characters has sat her down and had a talk about this.
And case comparison: to someone oblivious to interpersonal relationships, a total meltdown of a friend is far more obvious than amorous advances of an associate.