So all your relatives are coming for Christmas and your OH has only his mother who you 'don't want'. I presume your relatives are grown up and quite capable of realising what a ------------ your MIL is and treat her accordingly, maybe even bring her down a peg or too. Invite her , and if she comes pre-warn your relatives what she's like and give them advice to ignore her. Personally I doubt she'll come, as people like that prefer to moan about not being invited, or about being alone, than actually going somewhere and having a good time.