I had someone tell me this today and had never heard anything like this. They said if a male goat has a twin sister he has a 50% chance of being sterile or unable to breed. They also said that with Boer goat bucks, they couldn't be registered until proven. Not sure if it was only if they had a twin sister or what....lol! I asked another former Boer breeder and she said she had never heard this either.