You aren't the only one who feels this way, Just be the bigger person and be friendly. If only a few hours, it wont kill you to be decent. I have a relative like this, ( she's actually coming to stay this weekend for thanksgiving ) and I just smile and change the subejct when she does something stupid or says something cruel. Most of the people around you won't really catch you being rude, and they'd just overlook the obvious anyways with her, so your'e safe with that one. Just as long as you aren't mocking her, rolling your eyes or speaking too loudly, you should be able to grin and bear it.