I don't know.

I do have a friend who just had fraternal twins.
Her mother was also pregnant with fraternals, but lost one at 5 months.
Her mothers sister also had fraternals.
So I do believe its genetic to produce more than one egg, but I doubt any of that had to do with soy.

All the fraternals I know of have a family history of fraternals. It could be possible though. You never know.