Several months ago, I almost posted a spitball theory and decided it was too crazy even by my standards, but here I go. Note: I don't really believe this, but it does connect pieces. NOTE: When I say Brother in this, I mean Sensei's brother, not Sana's.
We know very little about Brother. We don't even know his profession. What if he was a teacher? What if Maya was his lover, not Sensei's?
What if Sensei's memory issues are happening because an external force is trying to overlay someone else's soul over his. Or more specifically -- Sensei is not the native soul to the body, he's the intruder. What if Maya is trying to use Sensei's body as a host for Brother's soul, but the result is unstable.
And every time the expiriment has failed, it's been because Sensei overpowers Brother. And this is why Maya was so freaked out about Noriko and Niki surfacing, because that's Sensei's past, not Brothers, and they could ruin everything.
So while Teacher was fooling around with Sensei, Brother was fooling around with Sara. And the reason why Sana's father vanished had less to do with him not wanting to be a father and more to do with his being dead -- but as a side piece, she wouldn't necessarily have been notified of that, would she? So she assumes he just walked out.
But boy, this Sensei that's teachiing Sana now sure does reminder her of the teacher that knocked her up, so that's why she fell so incredibly quickly. The fact that at least part of Brother's personality is also slipping through sped things along even more.
This might also explain why in Noriko's story, Maya just kinda shows up and Sensei immediately goes for her. Because that was really her and Brother resuming their fling.
Also this: Young Sara is small, slight of build, with dark hair. Just like Maya. Brother has a type.
I am %100 convinced it's a terrible theory, but I wanted to share.