Technicially it would be Gaara(even though Gaara is not a blood sibling of Naruto's) because they went through the same hardships as childhood, understand each other completely, and because they both have the tailed beasts inside them(which might make them blood relatives).
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he does have a sister tsuki she is a hidden member of the akatsuki naruto does not have a sister though in a few comics from shippuuden it is said that he does