Did you know that of the 43 men to be president, 30 were related to another president? That's 70%. That's a high number. In Ancestors of American Presidents (AAP), there is an extensive set of charts showing how all the presidents are related. Now, of course to do this, some rules were broken. The two most important presidents are Washington and Lincoln. In order for Washington to be connected to someone else, we have to delve into his gentry English ancestry. George was born in 1732 (the earliest of any president) and his eight great-grandparents are immigrants to Virginia. That means there are only 8 possible families for his to connect with others. (This is not impossible. Did you see this season of Finding Your Roots with Henry Louis Gates, Jr.? Senator John McCain is a Washington descendant). Likewise, Lincoln's mother's parents are unknown, so half of his ancestry is blank. So AAP provides several possible theories of ancestries for Lincoln.
The 13 presidents not related to anyone else are: Jefferson, Monroe, Jackson, Polk, Buchanan, A. Johnson, Arthur, McKinley, Wilson, Eisenhower, Kennedy, Reagan, and Clinton. And at least, Wilson, Eisenhower and Reagan married women with ancestries that tie to the other presidents. Most of these non-related presidents are of Scotch, Scotch-Irish, and Irish extraction. If we could do those genealogies back to 1600 like we can with colonial English ancestry, many of those presidents would be found to be inter-related as well.
The two most related presidents are the Bushes, Sr. and Jr. with 18 different presidential connections between them. This is not surprising since the Bushes have equal amounts of New England, Middle Atlantic, and Virginian ancestry.
The closest connection that is non-lineal is between James Madison and Zachary Taylor who were second cousins through the Taylor family. [Obviously the Adams's and Bushes are the closest related as father/son, then the Harrisons as grandfather/grandson]. The Roosevelts were fifth cousins by blood and nephew/uncle by marriage.
With this election we have the possibility of yet another close connection: husband/wife (if Hillary Clinton is elected). Neither she nor her husband are related by blood to any other president.