Well Soon to be graduate I am not going to say you are over reacting... BUT I am going to say pick your battles wisely. Making a battle over OW being in charge of D13 (including or excluding S16 in equation) could possibly backfire with you and D13, who may see it as a mistrust of her and not what you are intending-ie your concern over ow morals. Just a thought. Be realistic is it worth the aggro, will it achieve anything other than you "letting it out", will it bring your D home, will it benefit your sometimes troubled relationship with D13? Consider all the options before you act in haste.
You have been her mum a good long time now, whether she admits it or you are seeing it at the mo, she carries your values inside of her. Maybe allow her some credit to judge the situation for herself. What actual harm can befall her if ow is left in charge for the odd night? This could actually be a blessing in disguise.
Your her mum so it is your call just my thoughts as an outsider mum of 3.
