Who founded an American family notable for their social prominence, wealth, business enterprises and philanthropy, who has a member by marriage who isan American poet, writer, racehorse owner/breeder, socialite, and philanthropist ?  
Helen Hay Whitney:  She was a member by marriage of the prominent Whitney family of New York. Whitney family: The Whitney family is an American family notable for their social prominence, wealth, business enterprises and philanthropy, founded by John Whitney (1592–1673) who came from London, England to Watertown, Massachusetts in 1635.
John Whitney