I am having this same issue! My legacy heir in the generation before the one I am playing just keeps having more kids after I have moved out. And even though her house is full she is always in labor even though she just had a baby?? And one of my cousins is a single sim who does not have family oriented trait or anything now has 7 children all girls and when I load into the house to see what was going on she was pregnant and in labor again. And the same guy is the father to all the kids and it is a random sim that she knows nothing about of course.
