I would suggest hypnotic suggestion.
People are much more susceptible to suggestion than anyone would ever guess. It depends on your situation.. but:
So... after they're asleep (or before they're awake) you can open the curtains in the room. With the oldest, you can talk about lightness and alertness (even in really different contexts, it's surprising what is effective), being awakened by light, or the noise of other people moving around in the house, or some of the things that she's missing out on in the early hours... while you talk to the youngest about sleeping until she's had enough sleep, ignoring the light, etc.
It's reasonably helpful to talk to each separately because one message will dampen the other, and that makes a mess.
I suspect that the elder is feeling ripped off for 'having' to get up, because going to sleep late and getting up late are 'cool' on this part of the planet. If her sister can do it and she can't, she's going to be angry --angry people don't wake easily.