It is extremely irresponsible to allow 2 dogs from the same litter to breed together. 9 months is not an unreasonably young age to breed from but bitches should not normally be mated until their 2nd or 3rd heat.

The dangers of continual inbreeding include possible deformities and behavioral abnormalities. However, the chances of this are usually not much more than normal when this is the first case of such close inbreeding in the genetic line. You should take several responsible steps: Ask your vet if he is willing to be present at the birth in case of problems; have your bitch spayed after your vet thinks she is ready after her puppies have been weaned.