Unlike human siblings who are generally raised together, dog siblings are most often separated from each other when they are puppies because they are adopted or purchased by different families. The question of whether dogs recognize their siblings after being separated has not been researched extensively. The studies that have been conducted on kin recognition present stronger evidence for dogs recognizing their mothers than their siblings.
Psychologist Peter Hepper of Queens University in Belfast, Northern Ireland, conducted a study that revealed puppies at the age of 4 to 5 1/2 weeks recognized their siblings based on olfactory cues, i.e., smell. When the same puppies reached the age of 2 years, the then-adult dogs only recognized their siblings if they had lived together or been with their siblings on a regular basis. If the puppies were separated during that time, they did not recognize their siblings.
Hepper also found that puppies at the age of 4 to 5 1/2 weeks had a preference for their mother, even though they did recognize their siblings. Additionally, the puppies as 2-year-old dogs still recognized their mother when they were separated from her between 8 and 12 weeks after birth.
Hepper believes that the early life recognition is based on smell and that dogs retain the olfactory information for two years with their mothers. He also suggests that in the long term, dogs recognize their mother and siblings through different mechanisms. He suggests that there is a kin recognition gene or an experiential mechanism.
Hepper did not test whether or not dogs recognized their siblings between the 5 1/2-week point and the 2-year point, so it's not known when a dog's memory of his siblings starts to fade.