When two dogs born in the same litter mate, the likelihood that their offspring will have the same traits they have. This is why breeders sometimes interbreed their dogs -- it enables them to better predict things like appearance and temperament. However, it can also increase the offspring's risk of disease or disability, too.
Passing Common Traits
If two dogs who are siblings mate, it increases the odds that their offspring will have their shared traits. This is particularly notable when it comes to diseases and genetic defects, because most animals -- dogs included -- show signs of them only when the genetic codes they inherit from both mother and father carry the disease or defect. Every dog has two genetic codes: one from the mother and one from the father. A mother and father who are brother and sister may each have only one code that carries a disease, so they do not show symptoms. When they mate, though, their young may end up with two codes with the disease, one from each parent, so they will present the defect even though neither carrier parent does.