adversary: [n.] a person that somebody is opposed to and competing with in an argument or a battle. [Synonym]: opponent, antagonist, enemy, foe, rival, competitor
accomplice: [n.] a person who helps another to commit a crime or to do something wrong. [Synonym]: accessory, abettor, accessary, confederate, partner
sibling rivalry: a type of competition or animosity among brothers and sisters, blood-related or not.
