...and it reduces the gene pool to a little puddle.

Me too, Musick, I wondered about the inbreeding that would occur within a social structure that allows multi- marriages of male and female siblings, but after giving it a bit of thought I decided that Cavalli- Sforza’s account of events suffered during translation and what the book said wasn’t exactly what was going on in Tibet.

The way I interpret it, I don't think he meant a brother would marry his sister. I think he meant that a man (Mr Smith for example) could get a wife from a separate family (Miss Amelia Jones) and go ahead and marry all of her sisters (Julie, Elmedia and Gwendolyn).

So there would be no inbreeding involved.