Like I said, the impression I got, is that maybe she’s considering their feelings for each other as something other than platonic? In the subs she says “Yuu”, but if you listen to her she actually says “Yuu-chan” referring to the way Mika calls Yuu. So to me it’s as if she’s noticed how close they are and how important they are for each other. And I think she was looking sad because of her possible crush on Yuu and realizing there’s someone else he considers precious to him? Yeah, I know it may have something to do with her sister, but the way it played out between the mention of her possible feelings for Yuu, it just strikes me as something else. Just another suggestive scene if you’d like to call it that.