
If it's DDB's their gene pool isn't that small over here and there are quite a few health tested dogs with good scores just on this site. Actually there are a lot of DDB's advertised on here.
The problem is, that in some ways, the breed is still in it's infancy so you may not know yet what faults could be in these lines and doing such a close mating could bring them out.
The closest I've done is a grandfather/grand-daughter mating, I wouldn't want to go any closer than that and our breed has a much smaller gene pool than the DDB.