Call them Mira and Dan. Dan's mother lets herself in. Not often; she has a key for emergencies, and her definition of emergency includes dropping off soup. Mira has mentioned it. Dan has said “that's just how she is,” which is true and which is the problem, and last Sunday Mira came out of the shower to find her mother-in-law reorganizing the pantry and said something to Dan afterward that wasn’t about the pantry, and Dan said something back that wasn’t about his mother, and they’re now, technically, fighting about soup.
Why it’s never about the parents
In-law fights are loyalty fights wearing a costume. When Mira says “your mother let herself in again,” the question underneath is whose side are you on? When Dan says “that's just how she is,” Mira hears the answer: hers. He doesn’t mean that. He means he has spent forty years learning that his mother doesn’t change and it’s easier to absorb her than to fight her. But the marriage can’t absorb her. The marriage needs to know that when it comes down to it, Dan chooses the house he lives in.
There’s another layer. The way your parents did things isn’t a preference to you; it’s what normal looks like. Mira grew up in a house where you called before you came over. Dan grew up in a house where the door was open and calling would have been strange. Neither is right. Both feel like the only sane way to live, which is why the pantry felt like an invasion to one of them and like Tuesday to the other.
Whose parents, whose job
The rule therapists give, and it holds up: you handle yours. Dan talks to Dan's mother. Mira talks to Mira's. Not because the other partner is incapable, but because a boundary set by the in-law is an attack and the same boundary set by the child is a request. When Mira asks Dan's mother to call first, she is the outsider making rules. When Dan asks, he is her son asking for something, and she can give it without losing face.
This is harder than it sounds, because the partner who has to do the asking is usually the one who has spent a lifetime not asking. Dan would rather eat the pantry than have that conversation. That reluctance is worth looking at; it’s often older than the marriage.
The sentence that ends most of it
From the partner whose parent it is, to the other partner, before anything else: “You and I are the family. They're our parents, and I'm on your side.” Dan said it in the kitchen after the soup, badly, and Mira said “then act like it,” and he did, which meant a phone call to his mother that took four minutes and thirty years. The key is still in her purse. She calls first now. She was, it turned out, fine with it; nobody had ever asked.
When the in-law is the problem
Some parents don’t accept a boundary, and some are genuinely intrusive, controlling, or unkind. In those cases the sentence still comes first, because the marriage has to be united before it can hold a line, and then the line gets held together, with the partner whose parent it’s out front. A couples therapist is useful here in a specific way: they can help the child of the difficult parent see the pattern they grew up inside, which is usually what has kept them from setting the boundary for decades. The rule about holidays, by the way, is the same rule with a calendar attached; it’s in The blended family nobody warned you about, where it gets even more complicated.