The Clandestine Marriage

The Clandestine Marriage is a comedy by George Colman the Elder and David Garrick, first performed in 1766 at Drury Lane. The idea came from one of William Hogarth's engravings.

Warning: wikipedia contains spoilers

The plot concerns a merchant, Mr Sterling, who wants to marry off his elder daughter to Sir John Melvil, who is actually in love with her younger sister, Fanny. Fanny, however, is in love with a humble clerk, Lovewell, whom she has secretly married. Her attempts to extricate herself from the arrangement with Melvil lead to her becoming the proposed bride of Melvil's elderly uncle, Lord Ogleby. When the truth comes out, Fanny and Lovewell are forgiven.