I have been with my parents for almost a year now, my husband is still in another state and planning to move in with us soon. Mom only feels safe when I sleep next to her. She thinks Dad is coming in at night doing something to her. I tried explaining that Dad will not come in "her" bedroom, but she doesn't believe me, I know it's the dementia.
I would discuss this with her doctor, perhaps her meds can be changed to better suit her issues.
Hope that you can figure this out before there is no turning back.