Having read the original RAWs, I'd say that the original Japanese is just as ambiguous. She thinks "あたし・・・優にされてる時って・・・こんなかおしちゃってるんだ・・・", which pretty much translates to "So... This is how I look... When Yuu is doing me...".
As for what I think, as much as the sexual tension between them could be cut with scissors, I don't think they've gone all the way yet. Yuu's a bit too shy for that, IMO.
Concerning Mahiro x Mahiru, it's been canon that they aren't twins since the very first chapter they were introduced (ch14). They don't even have the same family name (Ogi Mahiru & Ooshita Mahiro), and this is also spotlighted in a later chapter about them.