Bob's Siblings are the five young puppies found in the crate with Bob. At least one of them is a female.
Ginga: The Last Wars
As very young pups, Bob and his siblings were places in a crate that had its lid nailed shut, leaving the pups inside without food or water. Over time, the pups grew weaker and weaker until one day they are forced out of the box by a bear, who flees at the sight of dogs. The pups are grateful to be released and seeing the bear doesn't frighten them. The bear, Monsoon, approaches the pups carefully and tries to scare them away but fails to do so. One of the pups licks Monsoon's paw then playfully nibbles on it. Monsoon sees this as an attack and crushes the pup under his large paws, killing it instantly and leaving no trace due to its tiny body. The rest of the pups are placed back in the crate and carried away.
As time passes, Monsoon tortured and eventually ate the pups, leaving only Bob. He gave the puppies scars using his claws and tore off the front paws of one, forcing it to walk on its back legs for a short time before he ate it alive. The other pups were forced to fight each other to stay alive but eventually each one was killed and eaten by the bears. Bob, being the only survivor, was forced to stay with Monsoon and work for him, even though he was deeply traumatized by what happened to his siblings.