Hmmm...to be quite honest, I think that she's going to just have to come around on her own. Look, she doesn't like you. And now you and her son have decided to stick it out for the long haul, as you put it.
So she's feeling not only upset now, but spurned by her son. So my guess is that nothing you can do will make her like you until she decides to let whatever feelings she has go. I know, it sucks, but its one of those odd facts of life. Sometimes you just have no say so in how other people feel about you.
Which means that you should stop worrying about it. That is not easy. For one, nobody likes being disliked. Then its your man's mother. Everybody wants mom to like them. But if she doesn't, no amount of goodwill, baked cookies, girls outings, nicely written notes, or hugs will do the deed. It requires change on her part.
So that end, here's my only advice to you: just keep being a good person and treating her son right (and he should do the same for you). That way if she keeps it up, she's just being petty and hopefully will see how unnecessary and foolish it is.
Good luck.
You cant change people, only yourself. My advice is be yourself, all you can do to people like this is to be polite and considerate. Be good to her son, let him make his own choices (with you in mind) and all is good.
If she choses to be the demon, dont follow her route. It will only strain your relationship with your man. Youer the bigger person, be the mature one.