I think the twins and triplets are random, so when I inherit I pick the peep that has the best combination of looks/career/desire to have children and no weird likes (like a guy liking makeup). Dislikes can always be removed. I've had lots of twins and triplets in a few of my families but none in one of my other families. I don't think the chance of having twins/triplets can be influenced in any way by the player.