Rajin and Fujin don't look anything alike, Rajin looks italian (or hispanic) and Fujin is pale. They are NOT siblings.
I may be wrong, but Fujin's features seem somewhat albino. I mean, her hair's grey rather than white, and her eye colour is difficult to make out, but I've heard of her referred to as an albino - not in the game, though. She's only 17 or so, meaning her hair colour is either very rare or else dyed.

She could very well be the same ethnicity as Raijin, just with different features. Besides, the duality of their names, attributes and personalities suggests a kind of 'kinship'.