Finally , in 1770 , when she was twenty years old , she was allowed to leave the convent and marry her younger cousin , the Louis Henri de Bourbon , son and heir of Louis Joseph de Bourbon , Prince of CondÃ and his wife Charlotte Ã lisabeth Godefride de Rohan . Finally , in 1770 , when she was twenty years old , she was allowed to leave the convent and marry her younger cousin , the duc de Bourbon , the son and heir of Louis Joseph de Bourbon , Prince of CondÃ and his wife Charlotte Ã lisabeth Godefride de Rohan .