I wouldnt do it, but if it did happen I doubt any serious problems would occur. But the problem would be if you bred the F2 generation to the F2 OR THE F1 there could be problems, overall just try to avoid it.
im not sure cuase i see brother and sister chickens mating all the time and the only genetic problem ive seen with my chickens is a chick with his head the wrong way but that was years ago