Help annoying soon to be in laws?

Well the problem isn't so much his mother but on sister and her children...My fiance and I have been together for 3yrs.I guess their major problem would be that I'm almost 30 and my fiance just turned 50. My problem is that these people are the fakey nice people. Ones who think they're not olny better than you but have the little snide remarks to go along with that attitude. It make family gatherings very annoying and uncomfortable for me. Family is extremely important to me so I've been trying to find ways to be diplomatic....it's not working. How to I tell them to shut the bleep up without saying exactly that?