I cannot find anything about Princess Francavilla Sun other than this portrait. Most of Roslin's works are from the second half of the century, but the style seen here with a split bodice and notched sleeves is from the first half of the century. According to his Wikipedia article, he was born in 1718. This could be one of his earliest works, possibly the one that brought him to the attention of the fashionable world. Or the sitter, whoever she is, may be dressing retro.