I know this isn't quite matching with yours, but I've only had a set of twins once. It was the luckiest roll of the dice I've yet had in this game. My sims had just used Ghost Potions (from Chemistry Lab)
Ghost Potion Temporarily turns a Sim into a ghost of old age for 3 hours. Gives a Sim the Temporarily Undead moodlet.
There's a chance that they'll have a ghost baby when they drink it and I thought it'd be a lot of fun.... and they had twin ghost babies! When they reached child stage, they started walking through walls!
That's my one and only experience with twins in this game.