Congrats on having a boy and a girl, even if they happened in separate pregnancies.
Procreation Elixir
$167
Wolfsbane Flower, Red Toadstools
Learned from Recipe Book. A Twinkle in The Eye - 8h moodlet that gives a great chance of conception and twin/triplet birth
From my understanding of this elixir, since it's only an 8 hour moodlet, to increase the chance of twins or triplets, it must be used within 8 hours of the birth.
Chances are better for multiple births using the fertility reward, and even better if you buy this for both parents.
Then if you have the mother listen to kids music while watching kid's tv, the % goes up dramatically.
As long as there's space in your house, of course, with pets that's a combination of 10 sims.
So with Georgie and Franky, Peter and Anastasia, plus Archie and Heather, that gives you 6 now.
And Tink the cat makes 7, so....you still have room for a set of twins or triplets!
But that's four or five children to choose heir from, even more hair-pulling than three!
And then you'd have to wait for space to have ghost kittens.
Decisions, decisions, decisions!