I have never heard of it either, but a very rude woman came to my job today told me about it. In the middle of a conversation I was having with a manager she interrupted to tell me how I need to be a cautious pet owner and not allow Teddy and Lola to spend too much time together because it could be deadly to them. I came home and googled it and it doesn't sound like anything "deadly" to me. Here's a link I found on it.
Littermate Syndrome: The risky downside to raising sibling puppies | Jeff Stallings, CPDT-KA