As Jody said, a lot of breeders will breed sons and daughters back to mothers and fathers. This is called line breeding. However, breeding brother and sister is not really recommended.
Very rarely does inbreeding cause mutations, contrary to popular belief. It can cause some health issues and may intensify some of the bad traits in a bird, but knowledgeable line breeding can be a wonderful thing in a breeding program. However, an accidental breeding between a brother and sister does not mean automatic death or mutation for the offspring. Typically, the offspring of such a breeding will be absolutely fine, but you may not want to continue breeding that specific animal because it's gene variation will be very limited.