Hi, I'd like to help you with your question.
I think what has happened here is that when your daughter was younger, she started sleeping with her sister and her fears became reinforced. So now that she is older, she sees her original fears as legitimate and can no longer remember how "safe" she was sleeping on her own.
You may want to try to have her sleep in her own bed in increments. Starting at a level where she feels safe, start slowing taking away the time she has in her sister's room. Give her any comforts she deems necessary such as the light on, door open, radio on, a special teddy bear, etc. Encourage her to extend this time alone as long as possible and offer her a reward for any night spent alone.
She has learned a fear and now does not understand that the only way to rid herself of the fear is to confront it. The only motivation she has to deal with this problem will have to come from you as her parent. Talk with her about her fears, reassure her, and then work out a reward system to help her have a reason to confront her fears.
You may also want to talk with her doctor about a referral to a therapist. Your daughter may need to talk with someone who can reinforce the ways you are trying to help her and who can also give you some ideas and insight into how to help your daughter.
Be assured, fears are easily treatable. With some help, your daughter will be able to sleep in her own bed.
I hope this has helped you,
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