The Prismriver Sisters are three characters named Lunasa Prismriver, Merlin Prismriver and Lyrica Prismriver, together appearing as the stage 4 boss of Perfect Cherry Blossom. Lunasa, Merlin, and Lyrica are collectively called the "Prismriver Ensemble" and perform at parties and festivals. At first sight, their instruments (Lunasa with the violin, Merlin with the trumpet, and Lyrica with the keyboard) may seem scattered and disconnected, but they still perform magnificent music. There was also a fourth Prismriver sister, named Layla Prismriver.
In Perfect Cherry Blossom, this is the main ability that all three sisters have been specified to have. This probably derives from the ability poltergeists have, put to specific use in performing with musical instruments. By manipulating the spirits of instruments, they're able to manipulate the spirits of notes to create sounds. Since they needn't have direct contact with the instruments, they're able to play music impossible to play by hand.
These three sisters are poltergeists, made by Layla Prismriver to have the appearances and personalities of her three older sisters from whom she had been separated.