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 lick each other, so the more subordinate members of a pack will lick the more dominant members and that's important in maintaining pack harmony.
Since 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 choose the strongest member of them and lick him as a submissive gesture to investiture him as a pack leader, like African wild dogs do.
However, in an environment that provides them food and protection like your house, usually they don't because there is already a pack leader nearby to them, which is you, and you are the one who they lick you most. Also, they lick others in your house, too, because others in your house are friendly with you and they look as strong and smart as you do, so the dogs show some submission to others in your house, too.
That is, unless they are known to be an aggressive breed or one of them loves to be dominant.
I'm not an animal expert, I'm just giving you some studies with some of my opinions.