Andre Norton 
Ralestone Luck 

The Ralestones were once a prestigious family. But now only three young Ralestones are left. There money has been squandered by a guardian. They have only one hope. Return to their old family plantation, Pirate’s Haven, and find the fabled sword Ralston’s Luck which has guarded and brought prosperity to the Ralestone’s for centuries. But their old homestead seems to be haunted and the ghost doesn’t want them to find the blade.