Does anyone know if there is some demon or entity connected with this?

Last night while staying over dog sitting, I heard the strangest thing. As I was trying to sleep I heard a woman's voice singing near my ear. I told my little sister, and all she said was "stop trying to scare me. . " when I told her I was serious she listened in and heard it as well. To her it sounded like it was coming from downstairs. It was a different for me. I couldn't sleep, so I told her to unmute the television to drown out the singing. The singing only got louder. Who knows how long it lasted. I convinced myself it was nothing and I needed sleep. My little sister apparently "saw" a girl peeking over the corner of the top of the stairs. Her description was pitch dark hair and piercing blue eyes. She has told me it possibly was a nightmare she had. . .she was to groggy to remember. ANYONE knows what is going on?