I use the multiple birth hack in my game and modify it several different ways. I also combine this with the family aspiration reward for increased chance of twins and the cheesecake twins cheat as well.
IVF treatments get increased odds for multiples as follows: 45% single, 30% twins, 15% triplets, 10% quads.
This is what happened with Ilila and Brandon Peterson, they received fertility treatments and their odds for multiples increased and they birthed little Jude and Brooklyn.
Advanced Maternal Age
Advanced maternal age (over 35 years old) get increased multiple odds: 55% single, 30% twins, 10% triplets, 5% quads.
Fiona and Jesse Kent fall into this category (though they also fit into other categories since Fiona and her mother are twins) since Fiona was about 35 when she conceived and gave birth to Jessica and Jennifer.
Generational Multiples-single case
If either parent to be or grandparent is a multiple: 45% single, 35% twins, 15% triplets, 5% quads.
Ray and Lydia birth twins Ari and Ava thanks to this, since Ray's father, Ajani, is a twin.
Generational Multiples-multiple cases
If both parents to be (or grandparents, or any combination of ) are a multiple: 40% single, 40% twins, 10% triplets, 10% quads.
Rebecca and Julio Wilsonoff birthed twins Abel and Amanda due to this. Julio is a twin, while Rebecca's father, Ajani, is also a twin.
Ginger and Grex Grey fall under this one multiple times. Ginger is a twin, her mother Cassandra was a twin, and her father Matthew was a triplet. Grex is also a twin. If these two didn't have any twins, I would be more surprised. But Annie and Bryant were their first and only set of twins.
What methods do you use to determine multiple births?