A very interesting point ....I just did a search and found this 
1.  Is in-breeding OK?  I know other animals from the same litter mating can cause mutations and stuff.  
Yes, inbreeding is OK as long as you are not breeding brother to sister (so OK to breed Father to daughter to granddaughters, & mothers to sons, grandsons and so forth. Be aware to that you must be careful not to inbreed a bird with a fault to another one with the same fault. What you are trying to do with inbreeding is to get the desirable traits of the line amplified.
2. At what age do roos reach sexual maturity? Breeds are different but actually about the time they start crowing, they are "capable of fertilizing but may not be efficient enough. I generally like to wait until my Sire is a year old. 
3. Do chickens mate only one time a year or do they mate whenever? They mate everyday, whenever.
4. Do chickens need a special pen to mate or will they mate where ever? No special pen is needed but I am told not to use wire floors. 
5. Will a young rooster mate with an older hen, say, one, two, years older them him?They will mate with whatever bird they can get. I heard of a young rooster covering a Cooper's Hawk once.
6. Will a hen automatically go broody when her eggs are fertilized?NO, broodiness is a hormonal thing. Some hens never go broody as it has been bred out of the breed or particular line. Some breeds are more likely to go broody than other breeds. The hen does not "know" whether an egg is fertilized. A broody hen will sit on nothing but air or say a golf ball. You cannot "make" a hen go broody.
So this poster says no to brother sister mating... Hopefully you'll get some more responses.  
Have a blessed day!