My wife and I got an unexpected visit from two people that own relatives to my dog.  One is a sister and the other is a half-sister (same mother, different dad).  They are both beautiful dogs.  Her sister has similar colors (black/white/gray) and her half-sister brown.   Her sister is 2 inches taller and at least 10-15 pounds heavier than my dos.  Her half-sister is about the same height, but much "thicker" in muscle mass.  I'm thinking that my dog was actually the runt of the litter.  We had our eye on her sister, but she was already spoken for, so we picked Sadie, who looked similar. When they were pups, Sadie was so much bigger than ther sister, but she forgot to grow as much.  Anyway, the dogs played for 90 minutes until they were all tired and ready for a nap.  Does anyone know if dogs have anyway of knowing their siblings?   I wouldn't think so.