Oh goodness, you poor thing! I don't know what the solution is though!
Are they into sticker charts? that might help with keeping them in bed longer in the morning, or at least in their room and playing quietly.
Does the older one like being 'responsible' and helping the little one back to sleep, if they see it as their job as the oldest? this seems to have helped my 8yr old dd go to sleep better, but her sister is 6 yrs younger.
failing any of that, I would be tempted to seperate them again