Maybe something like "Do you have more older siblings than younger siblings?" would work. One twin has to be born before the other.
That question has some problems, but I feel like it could serve as the basis for a more specific question that would define how an only child or middle child would answer without introducing much bias. It would also have to define how half-siblings are counted and probably some other things.
That question has some problems, but I feel like it could serve as the basis for a more specific question that would define how an only child or middle child would answer without introducing much bias. It would also have to define how half-siblings are counted and probably some other things.