Have you seen this article on twin language? http://www.slate.com/id/2302057/
My dad says my mom and her twin had their own language but I haven't asked her about that yet. She speaks perfect English, though. She and her siblings were raised as missionary kids in the jungles of Mexico, and she was just telling me that she and her siblings didn't really learn to communicate with their tribe of natives very well. Not like their parents could. They definitely grew up in isolation! Just interesting that they maintained their twin language until my father met and married my mother and he witnessed it!