DONT STICK DICK IN CRAZY SEI-CHAN
dont feel bad for mom, it was her own fault, are you all seriously ignoring all the things?
still attracted to mom is normal, that guy doesnt know anything, but he will soon enough!
I honestly don't doubt anything that the mom is saying. These are all things that I guessed about her, but I think the only reason she's saying these things is to manipulate Sei. She somehow knows he's in ear shot and is saying these things so he can feel sorry for her and go back to her.
Also if he leaves Fukiishi for her, I can see her going crazy too. And Sei as well because being so dependent on such an abusive parent will mess him up. If he goes back to her he will eventually hit the point of no return.