You’re wise to be vigilant, but her habits might be so ingrained that she may always think you’re overreacting. Point out to her that you have the right to react, overreact and underreact as you see fit with your children, and thus you get to decide what you want them exposed to. “Mom, I have strong feelings about this, and as their mother I’m going to lay down some ground rules.”
It might not be realistic to make her leave the house, but you can change the subject, not engage her, and otherwise drown out those lines of conversation. Finally, open a dialogue with your girls about this. Seeing you stand up for your principles and value their feelings will go a long way to counteract what she’s throwing at them.
Do you know what drives their lack of inclusion? Is there any reason to believe they think he treats you poorly? Is it some sort of prejudice? Is it a garden-variety personality clash? If the discrimination is truly unjust – honest friends can help you determine this – then you’re forced to take a stand. When he is actively non-invited, you can be a conscientious objector by not attending the event. Warn them of this in a general sense, before another invitation comes – letting them know you are committed to him, their exclusion of him hurts you and that it forces you to make a difficult choice. The longer you put up with it, the harder it will be to change the situation.