Middle school girl drama. Yuck. Over-nighters tend to turn into nightmares for at least one or two of the girls involved; just too much drama. Thank God I had boys! How about taking them out to a movie or dinner or bowling or something and sending them home afterwards? If you do let it happen, stay awake all night with them (at least in earshot) and supervise; they WILL get into mischief and/or squabbles; it's rather inevitable with that age.