The National Gallery of Art's note for this portrait explains why information about the sitters is hard to find, "The Frankland Sisters, another Hoppner group portrait, represents descendants of Oliver Cromwell. As a faithful spaniel sleeps at their feet, Marianne hugs Amelia, who holds a crayon and portfolio of sketches. During the Royal Academy’s 1795 exhibition, a newspaper critic noted that the picture 'does the Artist great credit: the Group is natural and graceful; the heads are sweetly painted.' Unfortunately, both sisters died of consumption at a young age."
I wonder when Boehner, Cantor, McConnell, Romney et al. are going to sell off the National Gallery's holdings to private collections for a penny, or less, on the dollar.
From Wikimedia. The image had a flawed band on the left side that was cropped and then spots in the background were removed with Photoshop by gogm.