A.C.
The idea of a transition period appeals to me. Could little sister have her own bed in big sisters room for a while? Or have a baby monitor in her own room to listen in on her older sister's breathing? Or could baby transition to sleeping in little sister's room in a bassinet or crib? At 5 months old, baby should be starting to be able to sleep through the nite...
When I was very small I slept with a very large teddy bear (I remember it being as big as me???)When I was a teenager I listened to a radio turned on very low to help me get to sleep.
When our kids were little we put on a Bible tape for them to fall asleep to. My daughter slept with a special (small) pillow until she was 9 or so. And she rediscovered it a while back and pulled it back into her life (at 16!).
Would any thing like this help your daughter? Ask her!
Good luck!