The daughters of LouisXV and Queen Marie Lescynska are a great topic for discussion, thanks for starting it! It always amazes me how much genuine familial feelings all the children of LouisXV had for each other and for their parents, what with growing up in the rareified atmosphere of Versailles, but they did, probably because of their mother. (Although Louis XV himself appeared to be a very devoted father, in spite of his many recreational pursuits.)
Madame Louise of France, the youngest daughter, indeed became a nun under the name of Mother Therese de Saint-Augustin of the Discalced Carmelite order. She chose to enter one of the poorest and most penitential Carmelite monasteries in France, that of Saint-Denis, rather than the more fashionable one in Paris. She ran from away from home to join the monastery in order to avoid a big fuss at Court. She later claimed her health improved as a nun for she was very happy there. Marie-Antoinette came to the monastery to give her the veil when Mme. Louise donned the Carmelite habit. When she made her final vows, all the churches were alerted and peeled their bells, spreading to churches in other villages, so that all the bells in France were ringing to celebrate the oblation of the King's daughter.