Ouch. That is a low blow ((((sunrise))))))
If I were you, and this is just my thoughts, I would make an extra effort to be nice to her - or the rest of his family for that matter. If you're nice to her, then you're essentially "rising above" her, and that will probably get more of a rise out of her than if you were to respond in a "negative" way like she has.
Also, the family can't be sitting around talking badly about you if you're being nice to them right? Don't give them the pleasure of that. Stand up to them and show them what a kind person and mother you really are.