I had triplets from this so listen up:
1 eat 2 apples
2 eat 3 watermelon
3 eat a hot dog
4 try for baby 4 times
5 wait till girl sim is nauseous
6 wait three days
7 have babies
(it worke dfor me to have triplets 2 times and twins 1 time so might be twins but hope hard and most likely will be triplets!)