James Madison, 4th President of the United States is George Washington, 1st President of the United States' nephew's wife's sister's husband!

George Washington, 1st President of the United States
 

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Colonel Samuel Washington
his brother

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Capt. George Steptoe Washington
his son

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Mary Jackson
his wife

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Dolly Madison, First Lady of the USA
her sister

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James Madison, 4th President of the United States
her husband