Having been convinced by my father that twins were due in my generation, but none of my siblings having had any, I read up a bit on the likelihood of me having them. I seem to remember being relieved to discover that twins are more likely the more pregnancies you have.
My maternal grandmother had twin boys (identical) at her 8th pregnancy (she would have been 35) and my paternal great grandmother had twin boys also at her 8th pregnancy (she would have been 38).
So I suppose the later mother's age would also explain it.......
Sally