If you mate brother to sister, or do other close breeding of related chickens, you should evaluate the chicks very carefully. Keep only those who seem healthiest, who grow fastest and whose feathers come in nicely. It might help if you keep track of who is who (use leg bands or those tiny multicolored mini zip ties / cable ties which you can get at a farm store/ hardware store , or put some kind of distinguishing mark on them with indelible marker and refresh it when it gets faded). Try to weigh the chicks once a week. This will give you the best clue of who is biggest, growing fastest, etc. Once the chicks are old enough to tell male from female, get rid of any runts (females who are smaller / slower growing than the other females, males who are smaller / slower growing than the other males). Get rid of any chicks with obvious deformities like a crossed beak, twisted toes, extra toenails, etc... (note, you are unlikely to see this kind of thing unless your chickens are already pretty inbred in the first place!) As they mature, get rid of any who seem to develop chronic health problems of any kind. You may end up being able to improve on the parents if you can "breed out" any problem genes which they carry. Good luck.