Every breed of poultry has been developed by breeding closely related stock wisely. The best pullets are bred to their sire, and the best young stags are mated to their mother and her sisters. That's how a strain is made and maintained.
Ideally, the breeder runs two families of related fowl and, when an outcross is required to keep vigour in the individuals, a suitable bird can be used from the other family.
If you have a good strain of a breed, doing all they should in terms of laying/table worthiness/conformation and type, you shouldn't risk it all going pear-shaped by bringing in un-related stock for breeding purposes.