Bad Manners and Brimstone

 I agree you handled it well.  While this doesn't look like something you can completely preempt. as soon as a relative starts in you can cut them off saying, "You'll have to take up any concerns about this with Fred.  I'm not discussing it with anyone."

Absolutely!
 Before I read this I was reminded of being in a similar situation.  I shut down both my sister and a distant in-law when they each wanted my opinion and more gossipy details.