When someone is that outrageous, it can get easier (though maybe not at the first dinner-It's hard because you assume the other people are not jerks).
From now on, you are forewarned and can go with whatever approach serves you best.
1) Bean dip, nothing but bean dip.
Ms Nosy "So is your husband tempted to cheat when he travels so much?"
You *stare at her calmly and bemused, count to 5 in your head, turn away* "Mom, how is your garden this year?"
2) Attack
Ms Nosy "So is your husband tempted to cheat when he travels so much?"
You "What an odd question. Do you often cheat on your husband?"
(warning: some people just answer these questions"