Ok--- thanks for the additional information, Michelle!! Now we can work on it!
So, first I would say that a bit of this could possibly be learned behavior; if she saw that your son was fearful and it was a big enough issue that gave him a great deal of attention, she may have followed in his footsteps... just absorbing the concept that sleeping in one's own room is scary. Important note: what we give attention to is what we will naturally see more of with kids... so if she gets your attention and some power with this undesirable behavior, it is working for her. You have now decided that it has gone on long enough and now is a good time to end it. As a positive reward, use one-on-one time spent with you doing something she likes. This way, she will get positive attention from you, which is something all kids need.... secure attachment and bonding time is a very valuable tool in parenting.
But, more importantly, we have to identify where her fears are coming from, what they are, and pull them out, one by one, at their roots.
I'm sure you've asked her what she is afraid of; it may be ghosts, monsters, etc... it could be that she thinks someone will attack her, or just the sounds frighten her. Cognitive-Behavior Therapy (CBT) would be very helpful to her--- to help her see that her thoughts are within her control and to teach her ways to change them and control the anxiety she feels; anxiety meds might also help, and it will be up to you both to make a plan together. If she can verbalize what exactly she is afraid will happen, you can then give her evidence that this is not true... for instance, if she thinks there are ghosts in her room, ask her if she is afraid of her room in the daytime, and if not, you can prove that it isn't actually her room, it's just the darkness along with the scary stories she is telling herself in her mind. To counter-attack those stories, make up a different story--- a positive one--- with friendly angels that are actually there to protect her, and you can name them, and make them wear silly costumes and sing silly songs, etc... and help her to see that she is creating scary pictures in her mind when, in fact, she has the power to create FUN ones, instead!!
You can begin this process by confidently deciding for yourself that you must now resolve the issue, and commit to following through to the end of it. Start by telling her that due to her age, it is now time to end this issue once and for all---- either with some changes to her room--- or with therapy/medication. Ask her what would make her more comfortable in her room, and if it is a new theme, an over-bed tent, a special lava lamp, soothing music in headphones, a book on tape she can listen to while in bed while she watches the lava lamp, a tv to watch and keep her mind busy until she dozes off... (and don't worry--- it won't keep her up all night--- she will fall asleep, and it won't permanently harm her to do this temporarily, just as a tool of comfort while you are working to ease her anxiety).
Tell her that you will allow her to decide the route to take. If she decides on changing the room, go for it. If she resists, make an appointment with a therapist. Any amount of surface rewards/punishments will just give unwanted attention to the unwanted behavior, instead of resolving the underlying issue, which is her irrational fear. The mindset and the fear of getting the jitters, a sick stomach, etc... from the anxiety is sometimes helped with medication, but I highly recommend that some CBT would be very helpful in resolving this issue.
I hope you find some of these suggestions helpful--- if not, just let me know and I will keep on working with you on it. Please let me know how this sounds to you... and, thank you for the additional information--- it makes a world of difference to get a full picture!