Question:  Katherine Stanhope, Countess of Chesterfield was the governess and confidante of this Princess who was married to who 
Sentence (from "Katherine Stanhope, Countess of Chesterfield"): Katherine Stanhope, Countess of Chesterfield (1609–1667) was the governess and confidante of Mary, Princess Royal and Princess of Orange, and was the first woman to hold the office of Postmaster General of England.
Relevant evidence: Yes