Unfortunaly the problem with siblings having puppies is that you are not only doubling up on the good genes, but the bad ones as well. Because she probably has a questionable pedigree with some health and temperment issues in the background, these are likely to pop up in the puppies.

There was a litter of goldens born here, out of littermates. I believe only one of the puppies wasn't put to sleep. Even at a young age these puppies displayed such strong aggression issues they couldn't even be touched. If one dog in the background had aggression issues, you are basically taking that one dog and doubling it up in the pedigree, so that the liklihood of aggression problems will be stronger. Same goes for health, and unfortunatly you have an unhealthy breed/s, very prone to joint disease, spinal problems, eye disease, etc