One morning in February, Matthew Allain, the senior managing director of the Leo Group, a private wealth-management company, visited a town house on Central Park West in order to discuss an unusual acquisition. The town house was the headquarters of Carpenter Fine Violins, a dealer in rare stringed instruments. It was also the home of Sean Avram Carpenter, the company’s thirty-three-year-old C.E.O.; Sean’s sister, Lauren Sarah Carpenter, twenty-nine, who is the company’s C.O.O.; and their brother David Aaron Carpenter, twenty-eight, who serves as C.F.O.