2002 Changes
Until 2002, the crown of Monaco could only pass to the direct descendants (including adopted children) of the reigning prince. This meant that Princess Antoinette was not in the line of succession and that Princesses Caroline and Stéphanie would lose their places in line at the moment of Prince Albert's accession. This created considerable concern that the throne might fall vacant and Monaco annexed by France should Prince Albert inherit the crown and then die without fathering (or adopting) a legitimate heir. In 2002, changes were made to the Constitution of Monaco which eliminated that concern.
Until 2002, the reigning Prince could adopt an heir who would succeed him if the Prince died without biological children. Prince Albert could thus have adopted one of his nephews, or even someone unrelated, and made the person his heir. That option no longer exists. Now, if Prince Albert II fails to father a legitimate heir, the crown will automatically pass to one of his sisters or one of their children—also regardless of any illegitimate children he has fathered.
Read more about this topic: Line Of Succession To The Monegasque Throne