[spoiler]Carolyn tells Julia and Barnabas the family all went away and that they would come back soon, but I don't believe it was ever mentioned where exactly the family disappeared to. Perhaps Gerard killed them somehow?[/spoiler]
[spoiler]We can presume that he was responsible. The family disappeared without a trace on the same night that Carolyn and Quentin went mad, and she most likely became the sole surviving witness to what occurred yet her mental status was too far gone to face up to what really happened so she chose to believe that they're away, and Mrs. Johnson must've bought into that too. Carolyn did mention looking for her mother's grave in what was possibly a fleeting moment of clarity (what a pity that Julia didn't have her medallion), and I'm sure that fear of reprisal from Gerard also played a part. Stokes told Barnabas and Julia that he wouldn't rule out the possibility that their bodies are still somewhere in the house (eek!). We saw David's headstone, but based on what Stokes revealed about that night, I don't know if we can be sure that he's actually buried there.[/spoiler]