Mothers-in-law know nothing. (Mine was a complete idiot.) They have proven since time memorium that their principle purpose is to cause discord in the relationships of their offspring.
Perhaps a subconscious — or even conscious — effort to keep their chicks from leaving the nest?
Don’t punch her. That would only cause problems between you and your husband — giving the Evil One exactly what she so desires.
Instead, swiftly end any conversation with your mother-in-law when she turns hurtful. And, in private, share with your husband what she’s said, explaining how it hurts your feelings.