Sorry to disagree, but the vet isn't necessarily right. There are a lot of variables.
Inbreeding is a tool used by knowledgeable breeders to fix certain traits in their lines, and to identify recessive genes for the purpose of eliminating them from their lines. In the past, before DNA testing, it was the only way to determine if there were recessive genes in one's breeding stock.
Inbreeding is also used to develop specific lines to outcross with other lines.
It is something that should not be undertaken lightly, however, and breeders who do this do so with a lot of forethought and research. It's a slippery slope, and fraught with complications. One must be very knowledgeable and do much research before attempting it.
In this instance I wouldn't do it. And without a lot more information I wouldn't suggest breeding either dog to any other dog, either. Will these two dogs be willing to breed, even though they're brother and sister? Most certainly. The best way to avoid accidental breeding is to spay/neuter them.
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