Treat them with respect. Don't assume that they're trying to make your life miserable. Set boundaries and hold to them firmly. Don't turn things into contests. Take the high ground. Don't resort to sniping or veiled insults. Stand up for yourself.
And if all else fails, move a couple thousand miles away so that when they do see you and your spouse, they'll be too busy being happy to see their child to make your life miserable.