Most of the time when I have twins in-game, I’ve used MCCC to adjust the pregnancy for storytelling reasons, which includes the gender too.
There was one time, a while ago, when I had the On a Ley Line lot trait and the couple who lived there had three sets of twins. This was before I used MCCC (or any mods). The first set of twins were girls, the second set were boys, and the third was one boy and one girl!