This is one of the most reliable fault lines there is, and it almost never looks like the fight it actually is. On the surface it is about a comment your mother made at lunch. Underneath it is about whose side you are on, and that is why it escalates so much faster than the comment deserved.
Terri Orbuch's long-running study of married couples found in-law tension to be a strong predictor of trouble, with an asymmetry worth knowing: what predicts difficulty is not closeness to the in-laws in general but whether the partner feels their spouse will represent them within the family. The problem is structural, not personal. You each grew up in a system with rules you never had to articulate, and now one of you is a guest in a place the other one calls normal.
The move that fails is defending the family. 'She didn't mean it like that' may well be true, and it is heard as a verdict — you brought a complaint and got told you were wrong about your own experience by the one person who was supposed to be with you. Note that you can validate without agreeing there was malice: 'that would have landed badly on me too' costs you nothing and concedes no facts about your mother.
The other half is the division of labour, and it has one rule: you handle your own family. The person who has known them for thirty years has standing that an in-law will never have, can be blunter, and can absorb the reaction without it becoming a permanent grievance. When the outsider raises it, it is an accusation from a guest. When you raise it, it is a family member being direct — and it spares your partner having to be the one everybody finds difficult.
Agree the rule out loud: each of you handles your own side. Then pick the one recurring thing that has been quietly costing you, and whoever it belongs to raises it before the next visit rather than after.
