I have to agree with Ginger's Mom about the "extra" parties. My parents and my sister's mother-in-law get along just fine and live about a half-hour away from each other, but each household has its own holiday celebrations. So my sister and her family spend time at each house on holidays--and they have their own celebration at home besides.

The not-getting-along part is a bigger issue, of course. And a very difficult one.