Millie and Melody Mouse are the nieces of Minnie Mouse. Although they can be full of mischief, they are sweet, just like their aunt. They are the female counterparts of Mickey Mouse's nephews Morty and Ferdie Fieldmouse.
The only known speculative theatrical film appearance of either of Minnie's nieces is in 1983's Mickey's Christmas Carol, where Mickey Mouse, as Bob Cratchit, has a daughter. In this film, Morty & Ferdie are said to have played Cratchit's two sons (including one as Tiny Tim), and since Melody seems to be the most consistent name used for any niece attributed to Minnie, it is quite likely that it was Melody who played the role of Bob Crachit's daughter.
In 2011, the twins would co-star in the Disney Junior series, Minnie's Bow-Toons. They first appeared in the episode "Trouble Times Two" where they visit Minnie and are shown to be crafty troublemakers.
Here, they first appear in the episode "Trouble Times Two". They've made semi-regular appearances since.
In Mickey Mouse Clubhouse, they first appeared in the special "Minnie's Winter Bow Show".