Missing the Obvious?

I feel like this article is not giving me the answer to a crucial question-- well, okay, a couple crucial questions. I feel like this article on the Indian village with an incredible number of twins, this research, is missing something big:

If they're a small and isolated village-- why can't it just be genetics?
Simple, right? A couple of twins moved in, or were born. Then they had twin children. maybe two sets. then those twin children had twin children, and so on down the line, until a majority of the village is carrying the genes that make twins more likely, and the place is overflowing with them. Why does it have to be something external, like the water? Why can't it just be the fact that perhaps there's enough isolation, and lack of mobility of the people within the village, that the twins stay there and have their own families instead of moving out into the world and throwing their twin genes away to far flung villages elsewhere?

Has anyone checked this out, before they started talking about something being in the water? The article doesn't say, but the fact that it's totally not addressed at all confuses me. You'd think it would be the first thing addressed and the first thing discarded. How many of these twins born every year are born to parents who themselves were twins, or had twin siblings, or twin grandparents?

It might be worth exploring.