Ever watch Honey Boo Boo? In lotsa families, there are members who are just socially awkward or who believe that family members have no boundaries -- It's okay to blurt out whatever comes into your head.
So what to do? If you reply "on the merits," as lawyers say, you'll simply be arguing with someone who is out of his/her depth. If you roll over, you can expect similar treatment in the future.
So I suggest the middle way -- As you listen to the person, put on the expression you might use if there was a booger hanging from his/her nose. When they pause for breath, reply mildly but firmly. Something like, "You know, everyone is entitled to his or her opinion, but adults choose an appropriate time and place and show some respect." Then just turn away.
Otherwise, holidays will be painful.