@scooter1171 Actually, there are two women: Akiko, the 'sensei' introduced in the first half of vol1 and the second half of vol2; and Sumire, the mother of Shou. I don't think we have seen Sumire since the mention of the divorce, the only female protagonist we see after that event is Akiko, who is married to the old man. Sumire is mentioned by the old man when Shou confront him but I don't understand his answer: either he says he pity Sumire for having a useless child, or say something about taking care of someone (Shou?) for 15 years (financially?). It would appears the one in kimono at that time is Akiko.
As far as I understand the son tried to have sex with the mother after seeing her cheating with the old man but he couldn't get it up when the old man forced him to fuck his mother and the mother left the husband and son to be with the old man. I guess the story has ended or close to ending with the chapter in Comic Magnum 110 and it seems at the end son opens a gallery himself and his mother comes to see it and they finally have sex but it there is a scene after that showing she is fucking the old man which I suppose is a flashback. Hopefully someone translates it or give a detailed summary starting from beginning since the story is pretty confusing .
In Comic Magnum 110, the woman who came to the gallery was 朱希子, not 今日子 because she was thinking about her student ,蒼太, when she looked at the painting. And the woman that 蒼太 had sex with is 吉井.