Hello and thanks for researching this very important question!
This is sort of a Yes-and-No question...
The practice of breeding related dogs is known as Line Breeding or Inbreeding. It is meant to increase the chances of making pups that have desireable qualities (appearance, size, temperament, etc.).
We have to be VERY careful when considering this practice today, since most dog breeds are already quite inbred (this is how dog breeds were developed from generic wild "dogs"). Line Breeding can be successful, but must be done with great care.
You must carefully consider the health and characteristics of the male and female in question. Are there any health problems in the families? Are there any health problems in the male and female? Are they gentle and tame pets? And so on.
Crossing two related dogs that may have a genetic predisposition for, say, Lymphoma, Aggression, or Hip Dysplasia, dramatically increases the chances that the pups will be afflicted. Genetic disease is the most common reason to Out-cross breeding dogs...that is, mixing males and females from distant (and best quality) genetic stock.
Many genetically-carried conditions do not show up until after the age of two years, so crossing young dogs can be a bit of a gamble as well.
You can read here about what the highest quality Boxer breeders in the U.S. are currently testing their dogs for:
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