If you do a search on line breeding, you'll get a lot of good information.
When a breeder is trying to develop a championship chicken or enhance a trait, it is not unusual for a rooster to be bred to his daughter or granddaughter. And there is genetically no difference in breeding a daughter-father than a brother-sister. The number of shared genes is the same.
When you do in-breed, any genetic weaknesses can show up, but good traits can get enhanced. Unless your chickens show some bad traits, you don't have a problem. And you can get bad traits from non-genetically linked birds.
Do a search on line breeding and you'll see what I mean. Don't just take my word for it.