Her marriage on 19 March 1619 was the last of the great Huguenot alliances. Her husband and cousin was Henri de La Tremouille, Duc de Thouars. Their mothers were sisters and their fathers first cousins to each other. 
She had been carefully educated and religiously was very well versed. She contributed as much as possible to the Huguenot party and dearly loved her husband. In December 1621, a son and heir was born: Henri Charles, Prince de Tarente. However, she took almost a year to recover from childbirth. 
When her husband deserted the Protestant cause, she was convinced that selfish motives and not religious conviction were the reason. Her son was taken from her to her own Catholic aunt, the Abbess of Sainte Croix, for his education and he was accepted into the Catholic Church, 
his education given by the Jesuits. However, after some years he escaped the Jesuit monastery and, under disguise, went to The Netherlands and the Court of Protestant Prince Frederik Hendrik. 
Her husband took the sons, but allowed her control over their two daughters. However, their daughter, Elisabeth, died when twelve years old in 1640. Many years later, her eldest son returned with a Protestant bride. After a short involvement with the Huguenot cause and imprisonment by Cardinal Mazarin, she secured his release. After this her son left France again. 
However, some years later her son sent her his children to be educated in France in the Protestant religion. Peace was allowed to last in Thouars until Marie died; but a month after her death, Louis 
XIV wrote and complained to her widowed husband about Protestant religious serviced held in Thouars. 
 

Source: Leo van de Pas