Especially between siblings of the same sex, fights can become severe, even dangerous, as they reach maturity. Siblings often play hard and fight harder. Because many sibling puppies only socialize with each other, they may develop aggression or fear towards other dogs. Professional trainers like me recommend not bringing home two puppies of the same age, let alone two from the same litter.
While this sounds like a good plan in theory, in practice it often causes a little distress and, potentially, some serious aggression problems. The most common reason given for not adopting two puppies from the same litter is that they will “bond better with each other than with you”. This makes sense intuitively, as puppies have already had the closest and most intimate experience with each other, and often during important phases of socialization. You're already struggling with the fact that you're an alien (also known as another species) and you're inherently confusing to your dog.
Most training professionals strongly recommend not adopting two puppies at the same time. The biggest challenge of adopting two new puppies is their tendency to bond very closely with each other, often excluding a meaningful relationship with their humans. In addition, owners often underestimate the time commitment required to properly care for and train two puppies; as a result, puppies often end up untrained and poorly socialized. When someone in the field of the “dog world” pastes their opinions in such a way that they have no supporting evidence, they should, in my “opinion”, expect a wider range of professional data.
Far away, with many hills and valleys where dogs could lose sight of the sheep and get lost. To maximize the use of its volunteer puppy breeders, a guide dog organization decided to try an experiment. I also found the article irritating and scolding and it didn't take into account the human influence on dogs, littermates or otherwise, nor the factor of being a strong pack leader. Training and going through the puppy phase with littermates is really more than double the work of a puppy.
On the other hand, I saved several puppies from my first litter for evaluation purposes and everything went well, for the most part. I have 4 litter mates, they behaved wonderfully with my training and with themselves; they are excellent in public and private in any combination of numbers, and I demand good behavior and training from them. Many owners of puppies adopted at the same time ultimately feel disappointed in their relationships with their dogs, even when they commit to providing for them for life. OMG I almost forgot, I also had litter mates before, with my last round of wd I raised one and ended up adopting his brother from a 6′ chain tied to a tree I walked past this dog for 5 years and never knew he was there.
But in working dog homes, where people are with dogs most of the day and there are several generations of dogs, I don't see why littermate puppies would be more problematic than unrelated puppies of the same age. The reason littermate syndrome was initially used is that it was a well-documented and researched fact that allowing littermates of two puppies from different litters to be raised together caused numerous behavioral and medical problems. Littermate syndrome (also known as sibling aggression or littermate aggression) is an anecdotal, unscientific term that refers to a whole range of behavioral problems that tend to occur when canine siblings (littermates) are raised in the same household beyond 8 to 10 weeks normal age, when puppies are usually placed in homes. It's just a matter of probability: there is a greater chance of problems among puppies (especially of the same sex) in the same litter than with puppies from different litters.
This allows him to concentrate all his efforts on that individual, with a fair wind and a good early socialization, he will take some of the good traits of the older and more experienced dog. . .