Gaaaaah. It's hard enough being a teenager and having to deal with the pressure to drink and use from friends - much less from adults who should be helping you navigate it all in as safe a manner as possible. I'm glad you were able to talk with her about it.
Can you talk with both kiddos and let them know that if, at any time, they feel uncomfortable, they can leave? Have them call you - or a trusted third party - to pick them up and bring them home? If she's afraid that having you pick her up will bring on drama from her dad and step-mother, does she have any friends that drive and that you both trust who could be their way to leave the party?