At the death, on 8 May 1768, of her brother and only sibling, the Prince of Lamballe, Marie Adélaïde became heiress to what was to become the largest fortune of France. Her marriage to Louis "Philippe" Joseph d'Orléans, Duke of Chartres, son of the Duke of Orléans, had been envisaged earlier and, while the Duke of Penthièvre saw in it the opportunity for his daughter to marry into the family of the "First Prince of the Blood", the Orléans did not want a union with an illegitimate branch of the royal family. However, the Orléans' mind changed when the Prince of Lamballe's death left his sister sole heiress to the family fortune. Although Marie Adélaîde was much in love with her Orléans cousin, Louis XV warned Penthièvre against such a marriage because of the reputation of the young Duke of Chartres as a womaniser.