When a name ends in 's', you add an 'es' to make it plural. Ex: Joneses. "I'm going to visit the Joneses." To make a name that ends in an 's' possessive, you simply add an apostrophe to the end of the name. You can do this whether it's a single person or a family whose name ends in 's'. But you must keep it consistent, according to the following article:
http://grammar.ccc.commnet.edu/grammar/possessives.htm