Catherine was Mary, Queen of Scots' mother-in-law. In fact, when Mary was five, she left Scotland and went to live at the French court, where she was raised by Catherine and her husband, King Henri IV. (That's Mary's portrait at age twelve, BTW.)
So, a little multiple choice pop quiz for you, just for fun:
Catherine and Mary, Queen of Scots
a) adored each other so much that Mary called her "my true mother."
b) despised each other so much that Mary, at her tender age, referred to Catherine as "that shopkeeper's daughter"
c) disliked each other, but were publicly polite to each other.
Answers below. Next fun fact:
Catherine's rival for Henri's affections was Diane de Poitiers, whom Henri openly adored, even though Diane was nineteen years his senior.
Catherine and Diane
a) got along quite well, actually; Diane was Catherine's lady-in-waiting, and lovingly nursed Catherine through many illnesses;
b) hated each other so much that Catherine refused to enter a room if Diane was in it;
c) were extremely fond of each other, to the point that historians believe the two women may have had sexual relations with each other.
And the answers are: