I definately don't see them as siblings. Although they seem to have a brother-sister like bond, I believe this is in line with the actions of the rest of the FF8 gang - as orphans, they have a fierce loyalty to each other, since they're the only family they have ever known. Since many of the Garden students' parents were killed in the Sorceress War, it is very possible that Fujin and Raijin were similarly abandoned, and possibly found each other. However, now I'm getting into too much speculation, so I'll back up.

I think that they are severely close friends, who could possibly become lovers, but aren't during the game. Why? The way I see the pair's relationship during FF8 is one of slightly more than friendship. Raijin not only respects Fujin insomuch as he defers to her wishes, but he also seems to be very protective of her; at the end of disc two he practically begs Squall to do something about Seifer when Fujin expresses that the situation is severely tirering her. Granted, this doesn't necessarily mean that Raijin loves her, but he practically follows her around like a puppy. Yet, I feel that during the game, his feelings are rather unrequited - which is why I don't believe they are lovers. There is someone in the way: Seifer.

Whether Fujin loves Seifer or not is really a moot point - either way she's fiercely loyal to him, and for most of the game is willing to sacrifice both Raijin and herself in order to follow him. This loyalty seems to come between Raijin and Fujin - in Squall's final battle with the pair, Raijin basically tells Fujin "I told you so" when they begin to lose. In response Fujin gets angry, but not soon after the pair finally give up on Seifer, refusing to follow him any longer. They become a united front, and I feel like Fujin has finally given in to Raijin somewhat. This decision on her part to put herself and Raijin ahead of Seifer and the "posse" allows for the pair to form a deeper connection. Whether they choose to pursue that after the events of FF8 is really up to the writers of fan fiction or FF8-2, if that ever occurs .

Finally, as for the whole Seifer/Fujin thing, I don't ever really see him returning her feelings, even if she ever had them. Besides, I'm a sucker for the whole Quistis/Seifer thing myself, but that's another story. . .