I suppose you could look her square in the eye and say "You're an adult; I have no desire to control you."
or you could say "I hope you're not meeting up with (OM); that would be incredibly disrespectful to our family on this holiday weekend."
But I'd be more comfortable advising you to do that if you had good intel that told you she was, in fact, meeting up with OM. Otherwise, you come off looking poorly if you're wrong, and -- as you say, if you're RIGHT, and you DON'T say anything (or if you say "OK, have a nice time!") you come off looking like you're enabling cake-eating.
