I don't think it's irrational for you to want to see that her son is as importnant to her as he is to you.
But I've been there done that and realized that you can't change people. Everyone deals with love and loss differently and for some people a relationship doesn't come as easily, openly or lovingly as the other person might hope.
I don't think that makes your anger irrational. But dropping pre-concieved ideas of how it should or could be and just trying to be grateful that it is at all, is the real challenge.
