In Kingdom Hearts, Sora has started his adventure unwanted after the Heartless invaded Destiny Island.
He then saves the worlds and Destiny Island is recreated (as seen in Kingdom Hearts II, where Kairi and Selphie are going to school and to the beach, also in the ending sequence of Kingdom hearts and Kingdom Hearts II).
When his adventure began, he wanted to create a raft with Riku and Kairi. When the Heartless came, there was a storm, so Sora went to the little Island immediately.
BUT here is the heart of the question: When he went to the island where the raft lies, a woman, presumably his mother called for him. Is this woman related to Sora ever mentioned again?
Kairis relatives were probably swallowed by the Darkness in Radiant Garden. About Rikus family, there is as far as I know never mentioned anything.
But Sora has at least one relative, which seems like he doesn't care for them or didn't had any bonds to them (which I can't believe, because he worships friendship so much).