Stressful In-Law Relations

We have an upcoming vacation planned and some unexpected (aggravating) news to bring our joy over our vacation down a few notches.  Part of it is going to be spent with somebody who hated me for years.  Sounds like fun right?  Instead of relaxing and enjoying a few days with family, I'm going to be stressed while spending a few days with family.  I figure by the time the vacation is over with, I'll need deep wrinkle cream (or xanax).

To cancel the vacation would disappoint my husband's parents (which I don't want to do) and based on hubby's "silent, moodiness" the last couple of days, he doesn't want to cancel even though he's not thrilled at the news either.  We'd reschedule the vacation, which was hubby's first choice, but I need to see my uncle on the first half of our vacation because it'll probably be my last chance (he's had cancer the last few years) so it's either go then because we can't afford two different trips.  Not to cancel the part of the trip with the difficult family member will disappoint me and drain our bank account to pay for an unhappy vacation (and probably end up with hubby and I fighting during vacation over his sister).  Oh decisions, decisions...   Here I thought vacations were supposed to be fun!!

Darned if I do and darned if I don't.  If I do, I need to figure out how not to let this person totally ruin the vacation and stress me out.