In 1718, Charlotte Aglaé began a romantic affair with Louis François Armand du Plessis, duc de Richelieu. In 1719, the duke was arrested and jailed in Hem in connection with his participation in the Cellamare Conspiracy. Charlotte Aglaé visited the duke several times in prison. Wishing to marry him, she urged her father, the Regent, to pardon him.
Her older cousin, Louise Anne de Bourbon, was another of the promiscuous Richelieu's conquests. The cousins, who had never been very close, became bitter enemies due to their simultaneous romantic involvement with the womanising duke. This enmity continued long after each amorous affair had ended. The young Louise-Anne was considered the most attractive daughter of Louis III, Prince of Condé.
In September 1720, Charlotte Aglaé caught smallpox and, according to her grandmother, the last sacrements were administered. She called her French confessor, Colibeaux, to her bedside, and, handing him a casket, directed him to secretly burn all the papers which it contained. Probably among these papers were the love-letters which she had received from the Duke of Richelieu.
In 1727, her former lover, the Duke of Richelieu, visited her in Modena in disguise, and the two resumed their romantic affair. When this was exposed, her husband allowed her to return to France in temporary disgrace. She returned to Modena later in 1727.