On the topic of rape does, Tsubasa want Sensei to rape her/Niki her(reffering to Otoha cuck scene)? She keeps meddling in Sensei's affairs and when he tells her to stop she asks if there are any consequences. Could this be her pushing Sensei to the point where he gets fed up to the point where he has her way with her?
There's definitely something creepy about Tsubasa and it's more than just "offering" her daughters.
The fact that she is offering them, and the amount of interest she has for him, she might be onto the fact that Akira's family must have some kind of special sperm and might want a child with him. Perhaps she is too old to conceive one so the task must go down to her daughters.
There's also no way that Tsubasa is oblivious to her "Tree" being special, specially if her family has had it for hundreds of years, she must know a lot about the "world" they're in and is looking to get something out of Akira, he is after all more powerful than he thinks he is.