[Kazu-kun]

Originally Posted by Bath-Teth

And why should the second eldest daughter (forgot her name) feel guilty about Sugimoto's problems?

To make it a bit more clear

Spoiler for manga spoiler for Bath-Teth:

Spoiler for just to be safe:

The second eldest, Kazusa, is engaged with Kagami-sensei, and they're soon to be married. In fact, Yasuko fell in love with Kagami when these two were dating.

All the sisters know that Yasuko is in love with Kagami. That's why Kuri called her bisexual instead of lesbian, and also why she asked Kazusa if she was feeling guilty.