In Greek mythology, Orthrus (Greek: Ὄρθρος , "Orthros") or Orthus (Greek: Ὄρθος , "Orthos") was, according to the mythographer Apollodorus, a two-headed dog who guarded Geryon's cattle and was killed by Heracles. He was the offspring of the monsters Echidna and Typhon, and the brother of Cerberus, who was also a multi-headed guard dog.
Cerberus was Orthrus' half-brother. 
The context states the relationship as being "brother", while I said "half-brother" - therefore, my statement is definitely incorrect because there is a difference between brothers and half-brothers (one common parent vs. both common parents). I think the system had difficulty because it does not understand the family structures and related terminology used by humans. 