I create a lot of twins in CAS but here are the ones I currently have who were born in-game (I haven't altered their looks):
I've got Lina (the one with glasses) and Nina Caliente (granddaughters of Nina Caliente and Don Lothario). Lina has twin boys born in-game as well (no mods), they're currently babies so I don't have pics of them yet.
I think a sim being a twin increases their chance of having twins.EDIT: I just realized that Lina and Nina have two moms and no dad so there's no way they were born in game! Also, their mother is Giuliana Bidot, who's the older half-sister of the two sets of Bidot-twins below.
Then I have Anna Kate and Aron Konrad Gilchrist:
And lastly I've got the Bidot twins, Cicero and Cosmo.:
They also had older brothers who were twins, Jacobo and Lorenzo.
EDIT: If anyone's interested, you can see the Bidot family tree
here