Marie first heard that she no longer had a living parent as she stood by the poor woman's bedside, and she was enabled, as much perhaps by the necessity incumbent upon her of attending to the wretched woman as by her own superior strength of character, to save herself from that prostration and collapse of power which a great and sudden blow is apt to produce. 
Marie's superior strength of character, and the need to attend to Madame Melmotte, saved her from prostration. 