WAG: Could twins appear to skip a generation simply because most people have more grandchildren than children, and thus there are more chances to conceive twins two generations down the road?
What they said.
Every ovulation, several egg cells ‘ripen’ in follicles of the ovary. Usually, the first one to reach maturity is released; the others self-destruct after receiving hormonal cues. Some women just have two or more eggs reach maturity close enough to the same time that they are released within 24 hours of each other.