If your goal is to maintain relations with them, then the safest way is to act as if you think every question is the start of a joke, and respond accordingly.
"Have you put on weight?" "Haha, what are you, the weight distribution police? Hahaha!" or "Oh, no, it just looks that way because I'm carrying the weight of so many judgmental assumptions. You?"
"Are you really having a potato with that?" "Of course -- doesn't everyone always have a potato with everything? Hahaha!"
"Isn't (DGD) walking yet?" "Oh, goodness, no, she's been far too busy reading 'War and Peace.'"
"You don't see much of your son, do you?" "What? I have a son? Where! Hahaha!"
"Why haven't you been promoted yet?" "Oh, I was overpromoted so high they had to start me back again at square one." or "I've been so occupied answering questions about why I haven't been promoted that I haven't been able to get any work done!"
The "What are you, the x police?" is a good fallback if you can't think of anything else. Works for every occasion. "What are you, the baby milestone police?" "What are you, the potato police?" And so on. Then laugh and immediately change the subject with a question of your own or the MrsFogis's friend method.