It depends totally on the genetics of the particular birds in question.
If they are from very unrelated parents, nothing bad is likely to happen in the first generation or two; if they are from a line that has been handled intelligently for years, with all individuals carrying or suspected of carrying deleterious recessives being culled, then nothing bad is likely to happen for a number of generations. OTOH if both parents happen to be carrying deleterious recessives (you have no way of knowing by just eyeballing the birds), there is a little greater chance of problems in the first generation, and more thereafter.
You may have to just have to sort of see what happens and act accordingly.
"Incest" is a totally human cultural concept, btw, has nothing to do with animals. And mating parent to offspring, or (less common for various reasons, but constituting EXACTLY THE SAME degree of inbreeding) brother to sister, is not inherently a terrible thing and in fact if people didn't do it on a regular basis we would not have stable breeds
You may find that you don't want to keep all the hens for breeding from and want to get other ones to add to the group, so that would decrease the amount of inbreeding overall, as well.
Good luck,
Pat