Pelléas and Mélisande
According to Caroline Potter, “The two sisters were both influenced by Debussy, and it appears they had similar literary tastes to the elder composer. Both sisters set poems by Maurice Maeterlinck, who was of course the author of the play "Pelléas and Mélisande" and also of "Princesse Maleine"; in February 1916, Maeterlinck authorized Lili to set the latter play as an opera. Allegedly, Lili had almost completed the opera before her death, though only the short score of act 1, scene 2, two versions of the libretto, and a sketchbook have survived."