in my opinion the mother knew what was coming.... Izumi said that her mother read through him. If we take that in consideration.
He knew when he was a child that he doesn't like girls. They where extremely close.
And the fact that the mother and the father where getting a divorce it was a good occasion to put some distance between them maybe.
But that's just my point of vue. I don't say that this is good. A parent that doesn't see his child is a terrible thing.
But the fact that she recognize Izumi and say that he has grown up but nothing more, was it a way to say to him that she has given up ? That she knows, she tried to separate them but she knows that it's hopeless to separate them. That they both want to be together not as brothers but as lovers.
I just finished ch 5 ,can someone explain to me why she didn't see her own child for 7 years ,and even after she saw him not a single tear or hug ????