Because of the potentially worst case (genetically) scenario of sibling impregnation. It's entirely possible for a 13 year old boy to impregnate a 12 year old girl. It's not rape, as neither child was able to legally consent, however it is incest. Siblings are the closest genetic relations you can have, and pregnancy between sibs is bad mojo.
No, father-daughter is closer and more common. And 90% of offspring are healthy enough to live normally. The inclussion of incest is unnecessary.