Who says it
Answer it in your head first.
Your spouse's family expect the two of you at every Sunday lunch and you cannot keep doing it. Who delivers that message, and why does it matter?
Their own child does. The relationship survives a disappointing message from a son or a daughter far better than it survives the same message from the person who married in, because in the second case it becomes a story about you rather than about a decision. It also matters because it settles the underlying question, which is whether the two of you decide things together and then each carry it to your own side.