The mother was mentally dominated and abused by him as well, so she's a victim. The father letting him be a monster is probably why she's trying to get a divorce to get away from them.
You're right but still... I have read the novel, in entire novel I didn't see the mother show any kind of emotions or love to the mc, nor any gratitudes after mc save her ass...
Why the mother also?
Mc's father was disappointed when the mc wasn't an Esper but guide. He couldn't believe his second child could be a guide, so he ask the mother to do paternity test, accusing her of cheating. This led constant fights between mother and father. Mc's mother was frustrated with the situation, so she ignored the mc, and his feelings, especially when mc needed her most. She couldn't even look at him because mc was "problem".
In the middle of the novel,
when the father confined the mother (because she wanted divorce) mc saved her, she didn't show any gratitude (too much attitude in her) Also when terrorist attack happened, mc again saved her life, yet she showed no concern for him.
In my opinion, mc's mother is shit.
MC's brother, father and mother all are shit