The Special traits of The Uzumaki are explained in the manga.
You guy's who've read the explaination stop making up your own damned idea's when some truth is already out in the open.
Red Hair, Longer LifeSpan and don't die lightly, Sealing Specialist's, Distant Relatives of the Senju Clan.
Karin has only been called "Karin" we don't know her Clan name yet.
Those of you who have read the manga know she came from the Hidden Falls Villiage from the flashback scene when sasuke encountered Karin for a brief second.
Yes Karin has some Trait's that resemble's Kushina's, but so does Sakura Personallitywise.
Naruto's never gotten to even Properly meet Karin yet how that'll go i don't know, but the reason for her reaction when she discovers that Naruto has an Evil Chakra Deep Inside Him(Kyuubi) could be Alot of reason's.
The only thing we can do is Sit Tight and wait for more to be revealed.
In japanese Customs it is possible for relatives(Clan members) to marry eachother, in this case a Ninja Fantasy World full of Ninja Clans.
It is Possible.
In the manga when Madara is to Retrieve the Rinnegan from Nagato he confirms that the Red Hair is an Uzumaki Clan trait, and that nagato was Still partially Alive.
Naruto is a bit Blind to subtle clues like the Red Hair trait, but if he were to start connecting these piece's together when or if Madara exposes this when he's to attempt to catch him.
Although i don't know how he'll do that when Naruto's already taken controll of the Kyuubi's Powers.