system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Can dogs remember their siblings?
gpt
Dogs can recall relationships. So, they can recall other dogs (or people, or other animals, or machinery) they play with, or generally have positive or negative emotions towards. Relation isn't necessary. Rather it is a learned response that needs a lot of repetition to enforce.

However, what is more interesting is that puppies learn a lot of behaviors at their young age. Just like children, puppies are like sponges. They really absorb a lot of behaviors. As puppies, they play with each other, fight each other, and learn many things from mommy dog. The puppies end up creating a lot of similar characteristics. They will have similar play styles, similar food relationships, same things to be scared of. They learned all of this from their early environment, their mom and each other. 

So it's no surprise that even if the puppies grow up, go their separate ways, and then happen to meet again in the far future, they might act like they remember each other. Most likely, this isn't the case. How much do you remember from your toddler days? Not too much. So rather, they are happy because they met another dog that plays exactly like them! And sniffs exactly like them! And humps exactly like them! They might even smell a lot alike! It's like magic. Imagine meeting a new person, and find out you both like playing all the same games, read the same books, enjoyed the same music and use the same body scent. It's insta-best friend! 

This is also why dogs from the same breeds seem to have similar play styles. Dobermans and Boxers are known for their jumping and flailing limbs. Border collies are all for playing chase. Huskies enjoy wrestling. Labs love to mouth. This is because these behaviors are passed from mother to children and decades worth of breeding also allows behaviors to be inherited, so to speak. Go to any dog park and watch dogs play. Dogs naturally pair up with dogs with similar play styles, and often times group up with each other based on similarity of breed.