Invitation Letter for Family
Most likely this person will be applying for a B-2 tourist visa. The U.S. Department of State provides an excellent overview on B-2 Tourist Visas.
Each U.S. Embassy/Consulate requests slightly different documentation for the B-2 tourist application, so please refer to the specific website where the B-2 visa application will be submitted. Most embassy websites will provide specific application instructions and list any required supplemental documents.