A.V.
Simply: "No, that doesn't work for our schedule. I know you want to sleep over often, but it's not possible. Please stop asking daily. I will let you know when it will be possible." If you are feeling generous, you can say, "You can stay til dinnertime" or whatever, but I think it's perfectly fine to treat her like any other friend of the kids and limit how much she is there. Many was the day where we said "no, but you can play/sleep over at x's house if the parents are OK with it."
If she has a hissy fit you can simply say, "Exactly. You need to improve your behavior if you want to be a guest in someone's home."
You don't even need to get into the whole business of who gets up when for what. It's not something you want to do.