Here is a question from my Ask Anne page …
I have a brother and sister from the same litter. Am I right in thinking that there is no need to get them neutered, coz they wouldn’t breed with each other, would they?
This is a popular misconception and my advice it to get them both neutered as soon as possible. The male at 6 months, and the female 3 months after the start of her first season. Be sure to keep her well away from other males and your own male at this time as he may still be sexually active for some weeks after neutering.
They certainly would breed together, which could produce severely inbred puppies which could be born deformed, deaf, blind or have internal problems.
Neutering also comes with health benefits and helps prevent conditions related to reproduction, such as testicular cancer, dominance or territorial behaviour and prostate problems in dogs and mammary tumors and pyometra in bitches.