I think you're right; I will probably just "know" when I've crossed this threshold and she may in time finally open up.
Having her son diagnosed as a schizophrenic (and the son who needed her quite a bit more than the average child) is hard shock for her, especially since he's quite nasty to her when he wants to be. She's still learning the kind of relationship and contact she can have with him while he's being the way he is, it won't be easy and I do empathize with her. Time will tell.
Bob, I wish you luck in your situation. It isn't easy but in the end we have to satisfy our conscience.
Resentment occurs when we aren't doing what we need to care for ourselves, though we expect others to do it for us.