Grandfather to Grandaughter in my opinion is not inbreeding but line breeding. We've had many good racing pigeons from line breeding. Mother to son, Father to Daughter, Brother to Sister, Sister to Brother would be deemed as inbreeding, this way has only ever produced birds of good type but not the greatest of racing ability, but there has been pigeons to break the rule.The best in my opinion is Cross breeding and line breeding. An interesting fact the best fancier last Season in The London North Road Combine and individual Points winner in East London Fed has no stock pigeons, but practices Line Breeding heavily, and the only crosses put to his birds are those youngsters bred for him to race, the best of these imports are then crossed.