I haven't noticed multiples having more of a chance of having multiples themselves. I had one heiress who was a twin and three or four generations after still hadn't produced any multiples. And in my current household both dad and mom are twins and they just had a single daughter (where there would have been double the chance for twins out of them). I think it's pretty random in the game but it might just have been a round of good luck for me in those saves.
Did your mom ever watching the kid's network while pregnant? That's supposed to increase the chance of multiples.