I HAVE figured it out. After looking at the census information and comparing verbal history, the truth slapped me. Incest had indeed occurred resulting in a mental retarded child being born. It wasn't in the brothers' family because there were only sons, no daughters born in their family. But one of the brothers had to deal with it in his marriage.
It all became clear to me as I was listening to another uncle speak of something that had happened and his way of talking clicked. Back then folks said things and used words that YOU would have to know the meaning of to understand the conversation. Such words as "had" or "know" instead of saying sex, "big" meant pregnant, "do 'way with" meant kill and so on. The brothers all lived on the same plantation so the children really did know who they were, just that they were told not to "have anything to do with" your cousin and they didn't. When we, the grandchildren came along, the brothers started teaching us the same thing but we were "allowed" to ask questions which my mother said they were never allowed to "question" anything. I guess the old saying that grandparents do things with their grandchildren that they never did with their own are true.
For years I have heard over and over the same thing concerning this particular brother but had never thought about what was being said, ie, he wasn't the father of the first born, the firstborn was slow, not right, had a condition, dumb, etc and that the wife was so "used to abuse and didn't know when she was being "used".
I choose to believe that these brothers had all the best intentions when they told their children not to "know" one another. I believe the brother told his other brothers what he was going thru and they came to the conclusion that they couldn't watch their children all the time so they had to give a command that wouldn't be broken whether sight or unseen. But they "failed' to realize that their children were misunderstanding them, or maybe the children did know what they meant but chose to keep it to themselves. I know for a fact that none of the other brothers ever said anything about the child and she was raised as a daughter, taking on the last name until she died. That is truly the big secret but they were trying to protect the family and didn't want to see it repeated again. It's useless to say that they should have been upfront with their children because, back then, that was NOT how things were done....