I believe it is because ur married to her son. Is he the oldest. B/c if he is then she is sad tht he is w/ u and not spending more time w/ her. She obviously still thinks of him as her little boy. you took tht away from her and she knows it and she knows it is not ur fault but she doesn't want to blame herself so she is blaming you. she is taking it out on u.
I understand y u would be mad @ her but i think you should talk to her and tell her how you really feel about the situation. She will respect u for it. She might even begin to love u and ur kids if she doesn't already.
She says and acts like she likes ur brother in laws girlfriends just to get on ur nerves. look beond it.
i would highly recomend u watch the movie Monster-In-Law