I don't think the bm is doing anything bad. The dad moved out and right in with his gf and her family. To expect the bm to feel good about it is a bit unrealistic. As is expecting the kids to be thrilled about the situation. Kids can figure out a lot WITHOUT there being any pas involved.

I think many people here would feel the same way as the bm does, and would've acted (as teens) the same way the writer's teen is.