N.G.
I remember having this same, crippling, irrational fear when I was a child. My parents were very stern and did not allow me to sleep anywhere else but my bedroom. I remember feeling the compulsion to look out the window every thirty seconds to see if someone was looking in at me. It would cause me to lose sleep, and just worry. Ultimately, I did grow out of it.
Why not just ask her what she thinks you could do to help to ease her mind? A relaxation CD may help- the noise can distract her and relax her at the same time. Have her listen to it on an Ipod or something. My daughter has lots of anxiety and I have her listen to music on her Ipod at night, seems to help her fall asleep.
Good luck, trust me I know what anxiety can do to a family!