Well, Raijin and Fujin are ancient Japanese Gods of thunder and wind, respectively. That's one of the reasons for the 'kinship' argument.

According to some sources, Fujin is only 17, which reinforces the belief that either (a) she's an albino or (b) simply likes having her hair dyed silver.

Personally, though, I think her hair is that colour because (SPOILER)it proves that Ultimecia is actually Fujin from the future.

...Just kidding.