William McKinley, 25th President of the USA is George Washington, 1st President of the United States' wife's first cousin twice removed's husband's brother!

George Washington, 1st President of the United States
 

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Martha Washington, First Lady of the USA
his wife

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Colonel John Dandridge, II
her father

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Elizabeth Wyatt
his sister

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Joseph Wyatt
her son

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Nancy Scott
his daughter

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Nancy Minerva McKinley
her daughter

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David Allison McKinley
her husband

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William McKinley, 25th President of the USA
his brother