Ralestone Luck by Andre Alice Norton

Rupert Ralestone is officially the Marquess of Lorne - but with no family money or prestige, the title is worthless. He and his younger brother and sister return to the old family homestead - Pirate’s Haven. Their only hope is to find the sword that was the family’s talisman for generations, the Ralestone Luck, and restore it to its proper place. If they succeed, the family fortunes will follow.