Listen, I've had it up to my eyeballs with the "Family Values" crowd. The unfortunate fact is, there are thousands and thousands of us who need to run like hell from our birth families. The thing is, when we're so emotionally involved our 'gyros' are off..it's difficult to tell up from down or right from wrong. An outsider can easily look at the situation and say, "What's so hard to figure out there? Obviously, the mother is a damaged, selfish person who will never be content."
I understand your position, but you need to be certain that you don't do something that you'll feel guilty over after things have calmed down. I'd say, you and your husband determine what you're willing to do and what you're not....what's reasonable and what's not, then let your mother know the new rules and stick to your plan. Just don't take her calls if she becomes harrassing. Maybe you can only see her a couple of times a year. Maybe all you can do is set her up with some sort of social services program. Maybe after you've done that, you'll still need to cut her out of your lives altogether. If you then feel guilty, just know that your perspective is distorted and get some counseling. Good luck to you!