Consider this list:
Charles Darwin married his first cousin.
Albert Einstein's parents were first cousins. Then Albert married his own first cousin. Elsa Lowenthal, Einstein's second wife, was his first cousin on his mother's side. In fact, they were also "double cousins." Lowenthal also happened to be Einstein's second cousin on his father's side.
Franklin Delano Roosevelt and Eleanor Roosevelt were fifth cousins, once removed (a chart showing their relationship is available at http://www.gwu.edu/~erpapers/teachinger/q-and-a/q6.cfm).
John Adams married his third cousin, Abigail Smith.