They were siblings and worked together when necessary. Together they killed all the children of Niobe, when they thought she had insulted their mother. They both loved their mother, Leto, and each other. Apollo protected Artemis's and her followers' virginity. Artemis would avenge Apollo if he was insulted.
Addition: The last comment has nothing to do with the question.
They are siblings, and usually they don't get along. They are opposites in their powers, and plus the whole you-don't-get-along-with-siblings thing. Artemis thought Apollo was irresponsible. Apollo thought Artemis was too mature.
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No, however in the myth of the war of Troy they are rivals for the Apple of Discord inscribed "for the fairest".
Yes, because as they're half siblings, their father, Zeus demanded peace. (Random answer :P)