Question about the title, how is it supposed to be translated? I think 義母 means mother by law (correct me if I'm wrong). Since we don't know how she legally became his mother, I guess she can still be a stepmother. We don't know what happened to the dad after all. But would foster mother also be written as 義母?
If you have lots of mums, you wouldn't worry about things like that. But if someone asks, it would be nice to know how to explain things.