It depends on what percentage of sexual contact constitutes rare, and where we draw the line. If cousins sexually experiment once or possibly twice, because they were curious, found each other attractive, and had the opportunity count as incest? Legally it does. Probably socially it does. But, if nobody ends up pregnant, they never partake again and keep the secret to themselves, who is to know?
Incest resulting in progeny is undoubtedly rare, but casual incest stemming from a combination curiosity, opportunity and perhaps a bit of alienation, is probably closer to uncommon than rare. Especially if we include cousins in the mix who generally don't grow up side by side.
