People often ask this about F and J.
Before they started speaking my language ("More chocoalte?"), they did what all babies do - they babbled. They weren't particularly advanced or different in this aspect of their vocbulary, but they did understand each other. Watching them communicate it did look a lot like they had their own language. They had many, many conversations and would quite regularly crack each other up. I tried long and hard to understand their babble but was never successful.
As their language skills developed and they started speaking English they gradually stopped having these coded conversations. They still have a far greater understanding of each other than many other people, but as they are siblings of the same age, at roughly the same stage developmentally, who have similar experiences and spend a lot of time together, I don't find this unusual.
As well as verbal language, all three girls know Makaton (a form of sign language). I've always used basic signs with them and as they got older they showed an interest in signing, so I did more with them. Knowing sign is great, but it is pretty useless if no one else understands you! Thankfully we know a few people outside of our family who also sign, including 8 year old S.
J and F are great friends with S. Their friendship is based on plenty of things other than Makaton, but their ability to communicate in this way certainly strengthens their bond.
S is quite a bit more advanced than the girls with Makaton. Sometimes this gets frustrating for them but most of the time S enjoys teaching J and F new signs.
This used to give me a warm and fuzzy feeling. Shared learning at it's best, right?
This morning we took Nelly to dog training. Everyone was happy doing their thing for almost an hour. Then F came up to me in her 'ready to go' mood.
I asked her to go and tell her sister it was time to leave.
I watched her walk over to J, who was talking with a group of mums and kids.
Being twins, they attract a fair bit of attention when they're together, and this time was no exception.
The whole group stopped and stared as F approached her sister.
I heard F speaking and saw her sign "Finished. Leaving." Aww... Warm and fuzzy!
Then I saw J sign, "Bullshit! Get stuffed!"
Thankfully, knowing sign is useless when no one else understands you.