I was going to take the time to rebuff each of the criticisms to my theory individually...but that would really take too much time.
First of all, even though it's the thread title, I feel I must repeat myself. I have not said that anyone is anyone else's daughter, mother, cousin, uncle or aunt. There is no information to support that. I do however believe that Karin and Kushina are unique individuals who have enough similar traits both physical and personality wise to be of some close relation. I believe this is why someone like Karin was brought into the story and why she still remains. It's as simple as meeting someone who reminds you of someone else...If you believe they might be related you ask a question.
Let me repeat this for the retarded folk...this is a theory. A theory I feel strongly about, but a theory nonetheless.
Regarding the relevance of character appearance in the manga. It goes without saying that physical resemblances are not an absolute indicator, but you would have to be blind, ignorant, or just a complete asshole to fail to acknowledge the obvious family resemblances given to the characters by Kishi. Honestly people, don't ignore the obvious, just to try to make a point...and yes, if another character showed up with pink hair, I would probably assume they were related to Sakura...just to piss you off.
Finally, I know someone will bring up the fact that I used the word obvious early on. If it were obvious...it wouldn't be a theory now would it. Don't be so fast to get sucked in because of a simple word that may have been planted just to set the ranters off. Much love.