I'm a certified Sign Language Interpreter and the way that GA describes the A family using the analogy of a hand is called "indexing". When you're interpreting, to save time and make your interpretation more fluid, you index oft referred to topics with your hand/fingers...example: Hold your hand palm out like the #5. First finger is GA, second finger is LA, 3rd finger is CA, pinky is Caylee and thumb is KC. Instead of fingerspelling the names, an interpreter would just point to the finger that represents whatever/whoever you set it up to represent. Like if you were talking about KC, you wouldn't sign her name, you would just point to the thumb or whatever.
I don't think that's what the A family did but that was the first thing I thought of when I saw GA make that analogy. Sure would make a good secret code.
(I hope that made sense )