I have often talked about this and watched as people stared at me like I was crazy. This doesn't always happen, but often a set of twins will develop their own language, such as my cousins Kate and Kolleen did. It resulted in the more dominant twin becoming the translator for the other, making it less necessary for her to learn to communicate to others in English. You would literally watch one tell the other and the dominant one communicate their needs. Of course, they were toddlers and none of it was exactly clear. But it was crazy to see, and a bit remarkable.
"Forty Percent of Twins Invent Their Own Languages.
These languages are called autonomous languages. Researchers suspect that twin babies use each other as models in developing language when an adult model language is frequently absent. The "language" consists of inverted words and onomatopoeic expressions. These autonomous languages are formed when two very close babies are learning how to speak a real language alongside one another and naturally often play and communicate with each other. While this is more common among twins, since they are more likely to be around each other and developing at the same rate, this phenomenon can also sporadically occur between two babies who are not twins. The made up "languages" often disappear soon after childhood, once the children have learned a real language."
-The Huffington Post