"Twins are especially likely to maintain an invented language because they spend so much time together and are on the same developmental schedule. They imitate and reinforce each other's early inventions, weakening each other's incentive to learn the mother tongue. They spend less time communicating with parents and other adults, on average, than do nontwins, because they always have a ready playmate and because their parents are especially busy. Twin parents must change more diapers, sleep less, earn more, and parry the brilliant questions forever tripping off other parents' tongues like, 'Is it true that twins only have half a brain each?'"
While I'm not sure my sister and I had our own entire language, we certainly had our...erm, pronounciation quirks that we mutually reinforced. The word "second," for instance, became "seck-UND" with, like, a really weird emphasis on the second syllable. And, the word for our mother somehow became "mop," always said with a special emphasis on the "p," after which we would howl in laughter.
My mom was not amused, and neither were our teachers. By the time we got to seckUND grade, we were purposely placed in separate classrooms and promptly put in speech therapy so we could learn to speak like ladies.
Nonetheless, a special place in my heart remains for being in on inside jokes, words, and lingo.
Talk about whatever you want in the comment section. Even if you're the snarky-ass anonymous troll who's been popping up on random fridays who is apparently too uncreative to think of a pseudonym! I have faith that You Too Can Be Agreeable. And if not, well, go shit in a hat.