I don't think Shura has a siblings relationship with Yukio and Rin. Or at least not from her point of view. Remember, she's the woman who flirted with and got rejected by their father. She also wanted to kill Rin up until they met. So she could hardly consider herself nor act like their sister back then and still probably don't.
But disregarding that and disregarding the age gap, I still don't think Shura and Yukio would work. Mainly because Yukio doesn't seem to be interested and because he seems like he needs someone that can soothe him, and all Shura ever does is anger him. Even though sometimes I don't know why he gets angry at her, but he does nonetheless. And it's not like I think everyone needs someone that's soothing, but I think Yukio definitely does.
Anyway, bottom line, I think Shura is often acting in a flirty manner towards Yukio (but she isn't serious about it; because if she was, she'd do way more). And if he took the bait, she'd probably reject him. Though every time he doesn't, it somewhat hurts her pride. But you hear stories like that sometimes, first it starts as a joke, but the more they get rejected, the more they keep trying, and the more they keep trying, the more they forget it was supposed to be a joke... idiots that is. X)
But whatever, I don't know anymore, since Shiemi said she's not in love with Yukio, I don't know anymore. X(