Cousin David is the nephew of Sam and Debbie Wolowitz and a cousin of Howard's. Since her wedding to Howard, Bernadette has become his cousin-in-law. As Howard and Bernadette's daughter, Halley Wolowitz, is his cousin once-removed. Howard says in "The Herb Garden Germination" (S4E20) that his cousin David is calling about the engagement ring he'd use to propose to Bernadette.