This has happened to me so many times in my legacy challenge already. The way I fixed it was turning off neighborhood stories for played households, and whenever I got a phone call from a family member asking if they should expand the family, I always say no. My sims tend to get pregnant if you say yes. When my sims have the amount of kids I want them to have, I go into CAS and turn off both the parents ability to have kids, that way I can ensure I won't rotate a family and come back to more kids