I can't suggested anything in the way of help for your sister but just wanted to add. Please Make sure you are clear & up front with your parents that you can no longer be the one there in the mornings. Don't agree to any "just this once" etc because it will just open the door to relying on you again.
Hopefullly your parents will remember how hard it is to have a baby in the house & not expect too much of you.