Princesses Caroline Matilda and Louisa by Francis Coates (image from Wikimedia). They were the daughters of Frederick, Prince of Wales, the estranged son of George II and the founder of all those Hanoverian father-son feuds that Queen Victoria and her children managed to carry on till the 20th century.
Louisa was initially planned as the bride for the Danish King Christian VII, but her ill-health meant her younger sister, Caroline Matilda, was chosen. I wonder if this double portrait was a kind of marriage catalogue for the Danish ambassador, or was just thought to be a charming display. The outcome of the marriage was filmed here.