Hi
I do agree that you need a bedtime routine, then cleaning teeth, kissing goodnight, going to bed, story, etc in an order become a habit. Im sorry but my boys haven't had to share a bed but they have shared a room. Being early risers, 5am, they tended to prod each other to see who was awake first.
I also have sat in the doorway just out of sight and every now and then say something so they don't see me but hear me and I sneak out and just come back for a comment and then find they're asleep. It's just persistance I'm afraid.
Maybe if the older one was told she could stay up 1/2 hr later because she is the older sister (make it sound like a big responsibility and fun) that may help. She might like to help put little sister to bed. Maybe a way to get single beds? bunks? divider?
I wish you luck, I can say my boys sleep like logs now and are reasonably easy to get to bed and once there stay put.