What happens if I don't know if these my faverolle pullets and rooster are siblings?
It sounds like you probably got them from a hatchery? Most hatcheries use the pen breeding system to maintain genetic diversity. That's where they have several roosters in a pen with many more hens and the random nature of breeding assures a genetic mix. The more chickens in that pen the better the mix, many hatcheries have kept their flocks going for a long time, though some occasionally mix in fresh stock.
There is a chance yours could be siblings or half siblings. They are almost certainly cousins. With a decent mix in the recent generations that is usually not a big problem. If they have defective genetics than those genetics could be enhanced, but that is the same even if they are not related. That's different from genetic diversity. Loss of genetic diversity is where they get so inbred they lose vitality, fertility, and are more susceptible to disease.
I'm not sure what you are breeding for: to show, productivity, behaviors, or appearance. Doesn't really matter. There are different ways you can proceed. One common method among breeders when developing the breeding stock is called line breeding. That's where you breed the best stock to each other to develop the traits you want. You can use siblings but they usually use a parent to offspring match. They can more point to certain traits with the parent-offspring as opposed to mating siblings. You do lose genetic diversity with each breeding but what you are trying to do is get rid of the genetics you don't want without losing too much stuff you do.
Once they get to where they want to go genetically they go into maintenance mode. There are different techniques for this. A common one is called spiral breeding. One common spiral breeding model is to divide the flock into three separate flocks, call them Flock A, B, and C. The hens always stay with their flock, the roosters rotate in a certain pattern every year. You might mate your best rooster in flock A to the best hen in Flock B, B rooster to C hen, and C rooster to A hen. Follow this sequence each breeding season. A good breeder can maintain good genetical diversity for a long time with this method. You can start with spiral breeding and skip line breeding. It might take you longer to get where you want to go (depending on where that is) but you will maintain better genetic diversity.
One very common model used on small farms for thousands of years is where you keep a closed flock (don't bring in any new genetics) for several generations, then bring in a new rooster or roosters to start the cycle over when you need to. The more hens and roosters you have in the flock the longer you can go without bringing in new genetics.
One very important thing in any of these methods, choose your breeders carefully. Be ruthless in only selecting your best stock to breed. Otherwise you are breeding defective stock.
I don't know how serious you are about improving your chickens through selective breeding. It can be a lot of record keeping and costs if you need separate breeding facilities. It helps to hatch enough chicks so you have more to choose between, what do you do with the extra boys and girls?
Probably a lot more information here than you wanted, but the bottom line is that I would not worry about them being siblings. If I hatched out some defective chicks I'd rethink that, but it is unlikely you will see any problems.
