If youve only a small gene pool,or you want to fix the best points of closely related stock,inbreeding is a valuable part of sustaining a good bloodline,only when you carry on inbreeding without the use of an outcross,do genetic problems surface.
I aint no expert mate. but surley if your breeding full brother to sister in lurcher or running dogs, not terriers or fighting dogs.
There going be a fault in either the dog or the bitch if you breed the brother to sister. then surley your going be twice as much likely to have the fault magnifed more in the pups.