Forget about it your first year. The inbreeding coefficient initially is no closer for sibling mating than Father/Daughter etc. That changes in the generations following. This does matter more when the source already has a high inbreeding coefficient. That is not the case in your situation.
Brother sister mating can even be used as a tool intermitted, but in a limited way, and with caution.
Vigor is huge in a production flock. A reason some are disappointed (sometimes) in more closely bred lines, and they may whine, when the solution might have been an outcross to birds of similar quality.
The most helpful thing that you can do concerning vigor is to hatch a lot, and select for it. That cannot be underemphasized. There is variability in the numbers alone. Hatching 30 chicks from a male or two year after year will tighten them up pretty darned quick. That can be good and necessary, and that can be bad.
Health and vigor is the number one consideration when selecting. The most vigorous birds are often (not always) the most productive. Vigor is especially important when selecting cockerels, because as an individual they have more potential influence. Mated in pairs there is equal contribution, but in flock mating or anywhere in between, the males will have more influence.
If I started with 24 chicks, I am going to keep 2 cockerels, and four pullets. I would mate the one male on the four pullets, and then the other. This is just in theory, because if I was not happy with any of the cockerels I would kill everyone of them, and start over. BUT . . .hypothetically, I am going to take the offspring in two different directions based on the cocks. From there, there is any number of breeding plans to follow. And I do, but not religiously, because I will not be so dogmatic as to limit myself. I always reserve the right to change my mind, and to do as I please on the side.
No matter the breeding plan, it is all for naught if good and vigorous birds are not selected. Like begets like, if the influence of the family behind the individuals is not considered.
I flock of mutts will become inbred very quickly if they are allowed to breed will nilly on the yard. Just because they are not uniform in type and color does not mean that they are not getting too close, and too quickly.
It is helpful to remember that the developed birds behind the commercial crosses are closely bred, and well bred. The parents are very high performers themselves. This is not to say that vigor is not important because it is. It is to say that well bred, and closely bred, is not necessarily synonymous with a lack of vigor.
All of it depends on the breeding choices. It is learning when, and when not to.
There are a million breeding plans out there. Pick what you like. Personally, to start out, there is no harm in flock mating. Some might gasp, but production flocks generally were and are. There are even very successful exhibition breeders that practice it, or some variation of it.
No matter what is chosen it still boils down to choices, and selection choices.
