I've often heard about twins forming their own "language", but I've always assumed that it only applies to identical twins. Is there any research out there on the subject? Does it apply to both fraternal and identical twins?
Here's what I saw with my fraternal twins:
While learning to speak, all children develop some of their own dialect. Some words they will use incorrectly for a while. Many they can't pronounce correctly, so they say them differently.
For a singleton child, this usually gets corrected pretty quickly, as everyone around them is using correct language all the time. If they want to be understood, they need to use correct language, too, so they strive towards that.
With twins, they learn from each other a lot of the time. If they can make themselves understood to each other, that will support their dialect. So it persists, at least for a time.
One of my twins spoke early and very clearly. The other knew just as much language, but had a very difficult time with consonants, so most people couldn't understand him. He ended up with his own, vowel-based dialect. The clear-spoken twin would translate the other.
Yes, twins can develop their own language, but it's not a given. They may, or they may not. Don't worry either way. I'm convinced that it doesn't matter at all whether the twins are identical or fraternal.
I'm an identical twin. I never had a baby language, but then again I grew up bilingual so there were plenty of languages in the first place.
Even with our identical twins we missed the twin language. I have not found proper proof that there is such a thing as "twin" language. So my answer would be that based on my own experience and document search, there is no such thing as a twin language. Causal examples should be tested with same age children that interact on a daily basis.