[QUOTE=kc261;1012017]Do you have any sort of source on that? Or personal experience that leads you to believe this?[QUOTE]
You have to take into consideration that either the father or mother has a different father/mother than the offspring they produce, so the only similar genetics are of whichever parent you breed the offspring to, when two siblings however are bred together they have no genetic diversity since they have both the same mother and father. Although it most likely wouldn't cause concern for problems breeding sibling to sibling, you still have the higher chance of a problem occuring.