T.P.
Have you ever considered putting her crib in the room WITH her big sister? It could be just that she would like to see someone in the room with her in order to feel safe/comfy. That would also build a huge bond as her big sister would feel like she's a protector from the monsters. You shouldn't think you're burdening your 6 year old either, she's simply sleeping and being company for the little one. If she wakes up and her sister is in the room with her, you may be able to simply put her bottle in her crib and if she wakes up for it, she'll see her big sister in the room with her and go back to sleep. It's worth a try.