My Girlfriend's Girlfriend is a modern take on the old "old school" ideas about relationships, and it takes a twist on the idea that guys really don't want their girlfriends cheating on them! David, your girlfriend's older half-sister, works as a baby-sitter. Everyday David and his half-sister, Corin, get a call from their half-sister who lives in Ohio. David has been trying to get his girlfriend to stop calling but she seems to think he is playing games..