When breeding dogs there are three main ways, outcrossing (using two unrelated animals), linebreeding (using two animals with a common ancestor in their pedigree) and inbreeding (using two closely related animals such as mother/son, brother/sister).
Although not ideal, there is actually no reason why inbreeding cannot take place, although any faults that are present will be doubled up on, as both dogs are from the same lines. Line breeding is the best way to maintain certain characteristics, but again unless you also know of any faults then you may be in danger of repeating them
Outcrossing means you have no way of knowing exactly which features may be passed on and most pedigree dog breeders use line breeding as a general rule, but the closeness of the linebreeding depends on the size of the gene pool and the availability of dogs.
It is a case of knowing your dogs' pedigrees and what is behind them, and breeders spend many hours considering which dogs will complement which bitches, rather than just using the dog down the road because it happens to be the same breed. (And in which case you may not know if you are inbreeding, because it may be brother to your bitch!).