A friend said that she usually slept with her brothers??
From the ABC video http://abcnews.go.com/GMA/video/missing-tucson-girl-fbi-steps-involvement-isabel-celis-16208983 the reporter says:
sources tell ABC News Isabel often spent the night sharing a bed with siblings, but on the night in question, she was alone in her room.
So we still don't have the 'source' imo.