Exactly. All those instances has the sibling relationship stated. The only way it'd be safe to assume that Raijin and Fuujin were related, would be if they either looked very much alike (if both had blue hair and Asian features) or if it was expressly stated (like if Zell saying "I cant believe those two are related"). As it is, the game doesnt even hint at a brother/sister relationship, through either appearance or text, so the chances of them being siblings are very very slim indeed.