Complex sentence: While in France, she tried her best to secure most of her daughters the best marriages possible; her eldest daughter was engaged to marry Louis Jean Marie de Bourbon, duc de Penthièvre and hier to the vastly wealthy "House of Bourbon-Penthièvre".
Split into simpler sentences: While in France, she tried her best to secure for her daughters the most advantageous marriages possible. The duke was the heir to the vastly wealthy "House of Bourbon-Penthièvre".