I think either of them would resent it if Shiemi got with the other. Rin would handle it better, but it would still sting.
If Rin and Yukio ever reconcile and decide to run off together, I hope they dump her for exactly that reason. I know she means well, but if there's one thing that can interfere with the twins' efforts to get closer, it's her. And they really don't need that.