Littermate Syndrome can occur between canine littermates or puppies who are not biologically related but are the same age and brought into a home together at the same time. Over time, the puppies develop such a strong, unhealthy attachment to one another that impedes their ability to bond correctly with their people or other pets. As a result, they never reach their full potential and usually exhibit poor behavior toward one another that can last throughout their lifetime.
Being in the trenches of the canine rescue world since 2005, at times I'd wonder why the rescue I worked with, before Woody's Place, refrained from taking on two dogs who had lived together their whole lives, besides the obvious reasons, of course....double the food, double the vet bills, double everything. But even if all expenditures were covered, the rescue would still shy away from the request to take littermates collectively, even if they were older dogs.
Well, after taking in Jensen and Jared (in 2019), two nine-year-old German Shepherds who have lived together since they were nine weeks old, I get it now. Before them, I had no experience with Littermate Syndrome or how hard it could be to spin the negative that comes with it into a positive. But I'm persistent if anything. :)