If your sim is watching children's TV they greatly increase their chances of multiples! I tried it when the game first came out and had triplets. I watch my sims carefully now to make sure they aren't watching the wrong channels when pregnant. I have had a family that had at least one set of twins in each generation without the TV but that's pretty rare. Make sure no TVs tune in to the kid's channel while pregnant and that may help. I don't know about the mod though I'm just talking vanilla game.