Krishna and Arjuna were first cousins. But their relationship goes much deeper. Sri Krishna reveals in the Mahabharata that they are Nar (human) and Narayan (divine) two sages in their past life who have fought the battle of the Ages for the victory of Truth upon earth. In the Gita Sri Krishna as the Divine reveals that Arjuna is His vibhuti and a chosen instrument to carry humanity one more step forward in its evolutionary march.
Affectionately,
Alok Da