How does Nick know Daisy and Tom?

Answer: Daisy is Nick's second cousin, once removed.

Explanation:

Nick, Daisy and Tom are characters from The Great Gatsby, a novel by F. Scott Fitzgerald.

Nick knows Daisy and her husband, Tom, because Daisy is Nick's second cousin, once removed. Moreover, Tom and Nick went to college together at Yale, but Nick was not very fond of Tom back then. Nick says that he spent two days in Chicago with them, shortly after the war ended. Nick and Daisy are not very close, but are reunited at the beginning of the novel, when Nick moves to West Egg.