Breeding brother-sister is absolutely the exactly same degree of inbreeding as mother-son or father-daughter. Mind, brother-sister is less often done than the latter two, largely because parent-offspring breedings have the big advantage that ONE of the parties is well grown and probably progeny-tested and thus you have better assurance that they're what you're looking to breed from.
From an inbreeding standpoint though it is exactly the same genetic thing, brother-sister or parent-offspring.
If you are just wanting a backyard flock, not really breeding 'for' anything, it's not such a terrible thing, just know you *might* get a higher incidence of poor birds in the ensuing generations. Inject fresh blood periodically to offset.
Pat