Page 20.
I remember seeing it on a future chapter, the reason why Amai "doesn't talk about her family much" isn't because of that. It's just because she doesn't have much to say.
If Sensei asked about her father I think she would answer, but apart from that, what does she can even say?
In Page 31 he comments about how she "never looks back", and that was the reason.
Before meeting and getting involved with Sensei she was dangerous close to dying. Even if she didn't killed herself she was almost having a mental breakdown. But he resolved her own problems by herself, even helped her father overcome his, Amai is doing well now. I think the reason why Sensei thinks like that is because he has a few unfinished business about his past that he never overcame, so he is projecting a bit, maybe?