Montana Skelton is a famous pop star who, along with her sister Romi Skelton, is part of the pop duo The Skelton Sisters. They are about 40 years old and are both apart of the Midnight Sun. Ms. Mauvais uses them in the second book to sing the Homunculus's song in one of their concerts so that they can capture him. She makes them sing again in the fifth book for Lord Pharaoh.
The twins have their own brand called Twin Hearts, where they sell things like lipstick, (which Amber collects and gives to Cass when she's half-way done with them) t-shirts, and their most notable merchandise, Sock Roaches. The Sock Roaches are actually the same thing as the Homunculus Cass had sewn, and this leaves Cass angry at the fact that they stole her idea and make money off of it.
Max-Ernest usually mistakes the Skelton Sisters's name for The Skeleton Sisters, although Cass remarks that they might as well be called that because they're so skeletal.