Littermate syndrome is real, and can have a dramatic negative impact on siblings when they’re purchased or adopted together. The bond between the 2 dogs becomes neurotic and strong. Humans become irrelevant. They can be impossible to train because their desire to cling to the other sibling overpowers their ability to focus. As the dogs reach sexual maturity, they can often begin fighting brutally. It can sometimes be resolved but it requires a tremendous amount of time and effort. In some cases one dog may need to be rehomed.