Just an interesting note that surfaced on another post reference.
While all pups in a litter are obviously half of their mother (obviously - duh), they may not all have the same father. Each pup is a separate ovulated egg (I don't know how common identical twins - split eggs - are in dogs. I've never seen one that appeared as such. Might be a good topic for another post). And depending upon the circumstances, and how much Mom got around (mixed breeds are often strays), there could be littermates who are only related by their MOM - if you know what I mean!