first Dogs can recognize other dogs as they are dogs.
There have been studies where a dog can pick out a dog in a photograph,"
Jill Sackman, senior medical director at BluePearl Veterinary Partners says :
"They know a dog is a dog and they can identify their own species."
and upon that It's sort of a submissive gesture when they licks each other
so the more subordinate members of a pack will lick the more dominant members and that's important in maintaining pack harmony
because they treat each other as a pack not a family when they grow up
and if they are living in a the wild they will chose the strongest member of them and licks him as a submissive gesture to investiture him as a pack leader , like African wild dogs do
but in an environment that provides them food and protection like your house usually they dont because there is already a pack leader near to them which is you ! so they licks you most. and they lick others in your house too cause they friendly with you and they looks strong and smarter as you so they show some submission to them too .
unless they known as an aggressive breed or one of them love to be dominant
i'm not an animals expert i'm just giving you some studies with some of my opinions