I was wondering what is more important for dogs, territory or blood relation. Will dogs still be territorial towards members of their family?
I can provide you with my observations of my dog, although you might be looking for a more definitive answer. Hopefully this will still help.
I have a cattle kelpie cross named Bill who I've had as my faithful companion for the last fourteen years, since he was eight weeks old.
He was a gift from good friends. My good friends kept one of the litter too and this brother Vautin survived until late last year.
Before my dog's brother passed away, I visited my friends at their home where the dogs were born.
After not seeing each other for over ten years and despite their age, these brothers still fought for the right to be the boss - the alpha dog.
Despite being brothers, and despite that both were born on this property, it was clear who was boss.
As my dog's brother Vautin was "home" and the stronger animal, he maintained his role as the dominant animal.
That did not stop my dog Bill's attempts to take that role from Vautin.
Although harmless and no injuries were sustained, their fight for position was very serious.
So from my observations, in this case, I would suggest that territory is more important than blood.
However I note that dogs come in not only a wide variety of shapes and breeds, but a wide variety of "personalities". So each breed / litter / location could result in different observations.
Hope this helps.
Here are my two bits of info: Dogs are territorial.
Up to a certain age, (about 8-12 months in my experience), the dogs are young, and do not develop territorial and possessive behavior. After this age, its their biological and psychological need to establish their territory. Male dogs will start establishing this rule within their brood by being aggressive in what they claim to be their territory, they also begin marking their territory (if you know what I mean). Female dogs take a little while longer for developing this behavior.
However, fact of the matter is, whatever be the other dog's relation with your dog, brother, sister, mother or father, the dog WILL be territorial after a certain age, to all of them. No exceptions.