The Sea Witches are the daughters or nieces of Ursula and Morgana, and the possible sisters or cousins of Uma. They first appeared in the novel, the Isle of the Lost.
They play violins in the junior orchestra at Dragon Hall.
In Novel Rise of the Isle of the Lost, Uma is the only one of the main three characters (Uma, Gil, Harry Hook) without confirmed siblings. This could indicate Uma is an only child and that the Sea Witches are her cousins.