After your update, I think you need to stand your ground and move up girl who is ready and keep twin with the level 3s. We have girls of all ages at level 3 and level 4 and it's part of gym to learn to get along with kids of all ages, not just same age, much like in life. Many twins are at different levels in gyms all across the country. We have had several sets of siblings where younger one moved up quicker and it all worked out. I think if you give in now, you'll always be giving in every move up time. Won't do either twin any good.
Good luck!
Good luck!