I created two YA brothers to live next door to my sims. My sims' two YA daughters went over to visit them and got immediately married in one case, and engaged in the other case. All had family-oriented and the girls had Fertility Treatment too. The house was set up with rooms for babies and children, lots of skilling things and play things, I had all the bases covered.
Days went by and still no babies over there. Every time my sims tried to visit them, they were gone, at work. Finally, at 1:30 a.m. they were home, so the parents went to visit them. I had peered into the window and they all four were standing still in the empty house. As soon as they opened the door, they jumped into action and furnishings appeared in the house. The husbands immediately went to bed, but the daughters stayed up and visited with their parents.
Finally I realized why the girls hadn't had kids -- the brothers were sleeping together in the same bed. And I can't do anything about it!