[cynicalicious]

Pardon me for a second.... ah wait, there are no emoticons for pounding my head on the desk.

Spoiler:

The relationship between Sachiko and Yumi isn't romantic. The best way to understand it is how Sachiko describes Yumi-- 身内. It's a word that you use for your relatives. Sachiko doesn't just see Yumi as her "little sister," she sees Yumi as part of the Ogasawara family now. That's how their relationship is. So no, romance doesn't enter into the picture.

Same goes for Touko. Touko is now part of Yumi's family in her mind.

Yumi loves them both as her older sister and her younger sister. As 身内. Not as anything else. So no, they "are not fall in love each other." She loves them, but not how you're thinking.