My experience has been people who have litter mates have more difficulty bonding to the pups than if they are separated and then reintroduced at a later time. After 8 weeks the social skills that are taught in a litter are sufficient. Between 8 and 16 weeks the goal is too transform the dog into a home environment and to have it bond with you. This bonding goes on longer than 16 weeks but this is a critical time. If you have two littermates who only want to play with each other and not you, that bonding will never occur or severly hindered.

It doesn't matter if the dogs are siblings or not, when they reach a certain age if they have the dominant personality trait, they will try to establish pack leadership with any other dog who doesn't submit to their authority (sibling or not). Dusty, Dixie's son and 4 years younger tried my patience for about 4 months with his challenging for alpha status. It wasn't a real problem though because Dixie is alpha and put him in his place numerous times until he got it through his head he wasn't the boss.