Its really just not an issue with chickens. Most problems are caused by improper egg development. This happens when eggs aren't turned just right, or environment changes. Really if you consider the amount of chicks hatched compared to the amount of defects its very minimal.
I'll try to simplify what I was saying in my last post. Large scale barns house thousands of chickens. Just as in your backyard each rooster has his own flock of hens,he has a flock in the barn. The chances of you getting related chicks out of the thousands and thousands of eggs that are shipped our weekly is relatively slim.
All of this takes place in different barns. In one barn the chickens mate and lay eggs. The eggs are gathered twice, or three times, daily and are shipped from that barn, or usually several of those barns, to the hatchery to be incubated. They are then shipped to you. Your chicks may not be from the same tray of eggs, or from the same barn, or even the same farm.
However even if you were to breed siblings unintentionally it wouldn't have an adverse effect. Chickens are not complex animals, but I know that, particularly if these are your pets, it has a bad ring to it. In the farming industry line breeding has been practiced for centuries, most likely forever. This is a way of breeding superior animals by breeding bloodline back to bloodline. Its done with horses, cows and most any other farm animal I'm sure. Your chickens will be fine, even if they mix a little blood.
Hope this cleared things up a bit.
