No, we do not have our (original) twin language anymore. We do have a sign language deal we made up, part ASL, part nonsense. But that is not a true idioglossia or cryptophasia.
"Idioglossia" is a scientific term for twin language. "Idio" comes from "idiosyncratic," and "glossia" means "of the tongue." "Cryptophasia" is another term for twin language, "crypto" means "secret" and "phasia" comes from "aphasia," a speech disorder.
Words we still remember from our language (because our family thought they were hilarious and therefore still occasionally uses them):
Fogee, Fogi, Foji: pronounced "Foh-jee," Baby A came up with that one for our older sister
Our baby and toddler years were spent in Hawaii, so we picked up on some Hawaiian Pidgin as well. That had a hand in our pronunciation of things and our grammar. Our family still uses "all pao" sometimes for "all done, finished." Back then, we dropped the R or various other letters off of the ends of words. Apparently, one time when I got a plant piece in my foot, I went to my dad and said "I got one stickah in one foo." "One" is more commonly used in Pidgin than "an" or "a."
Our idioglossia was influenced by both English and Pidgin. The reason for an idioglossia between any siblings that are close in age is due to the natural mimicking and practicing of speech. When babies are together and practice talking, their mistakes get incorporated into their speech. It can turn into something that sounds like an entirely separate language.