What has happened to Sora's Family?

When Sora went to the adventure in KH1 he did it unintentionally.
There was a storm that swallowed him forcefully and send him to another dimension (also seems as world).
It happend because Riku opened the Door To Darkness and it led to unstable connetion between all of the worlds who connected to his home town.
As for your question, Sora was determined in all of his journey (KH1,2) to bring back his friend home in one piece (and he succeeded ☺).
He didnt have the privilege (because of promise he made with donald and goofy) or the time to think about his rest of his family.

So it's not like he forgot them or anything like that, he made his way one task at the time.