Private languages like that are called idioglossia. It's most common in twins (which is called cryptophasia). 

https://www.mnn.com/family/babies-pregnancy/stories/secret-language-cryptophasia

https://slate.com/human-interest/2011/08/cryptophasia-the-secrets-of-twin-speak.html

Wow, I didn't realise  it was a thing amongst non twin siblings. My 10 and 14 year old do it too, they have their own unusual way of speaking to each other and have for years. It started off as a game, but has gotten less and less understandable. Its a strange, high pitched kind of version of English with weird pronunciations and sounds pretty much unintelligible.

Me and my brother are Irish twins and we had our own language, too! At first it was just tone changes in our voices but people were still able to pick us up so I made up a vocabulary for us both to understand. Family picked up some words here and there but me and my brother could have full conversations in our language without anyone understanding us. 

We also developed a writing system, so we could send each other coded messages in our language. It drove my mother berserk! ?. I came up with new names for all of my family members as well but mine and my brothers names changed a lot. Ours is a high pitched variation on English and some Gaelic for good measure and to throw people off what we were saying. 

So lovely that you and your sister have a special bond via the language. I'm sorry to hear that others in your family were not supportive. My middle son has been working on a new language with a few friends for several years. 

On 5/25/2019 at 10:58 PM, ILoveJellybeans said:

Wow, I didn't realise  it was a thing amongst non twin siblings. My 10 and 14 year old do it too, they have their own unusual way of speaking to each other and have for years. It started off as a game, but has gotten less and less understandable. Its a strange, high pitched kind of version of English with weird pronunciations and sounds pretty much unintelligible. 

On 6/5/2019 at 10:06 PM, WitheredRose said:

Me and my brother are Irish twins and we had our own language, too! At first it was just tone changes in our voices but people were still able to pick us up so I made up a vocabulary for us both to understand. Family picked up some words here and there but me and my brother could have full conversations in our language without anyone understanding us.  

We also developed a writing system, so we could send each other coded messages in our language. It drove my mother berserk! ?. I came up with new names for all of my family members as well but mine and my brothers names changed a lot. Ours is a high pitched variation on English and some Gaelic for good measure and to throw people off what we were saying.  

Ok... this is baffling.... because my sister and I also have our language spoken in a really high pitch?! I thought it was a little weird, but here I have two people telling me they either speak or witness made up language that's spoken on a higher pitch than normal?! Our language began as play, it developed into a high pitch, then at some point we realised that switching to English based words would make our speech even faster and now it's 85% English and 15% Finnish based words that we speak. If one would really want to dissect our speech they could probably understand more, but to just regular listeners the way me and my sister speak is far too fast and obscure, sometimes the way we write is even far more obscure than the spoken word.

Ok, this is far too cool!