we can all imagineSadly, we know close to nothing about Riku's family. The only thing we can assume is that he at least has parents. He mentioned them very briefly in the first game when he said, "Once we step through, we might not be able to come back. We may never see our parents again. There's no turning back." Judging from the fact that he wasn't been home in about two years, he probably wasn't very close with his family. Hopefully Square-Enix will go into this further with the upcoming titles.