Re: LGBTAitP #31: Cuddles Are On Back Order. Have Some Snuggles!
Originally Posted by Asta Kask
You'll need a sample of people where both siblings are homosexual. Might be difficult to get enough people.
*skimming Wiki*
People have also looked at things like the Israeli kibbutzim for the Westermarck effect, though. I know I've heard of that before. So having the children be siblings isn't necessarily necessary, just raising them together from early childhood.