Just a thought, but considering that Nozomu and Sekai are remembered, while it seems like Yuu specifically isn't:
Yuu might have been Sara's teacher and the father of Sana and Shota. It could explain why the name Arakawa doesn't ring any bells for her. She can't actually remember who her teacher was, just that he existed.
Potentially, it could be the same with Rin, making her Sensei's half sister. This could also fit in with Nozomu possibly being gay. Their father was the slut this whole time, and Sensei might be Yuu 2.0. He may even end up just like Yuu: