What clubs in NYC are 18+ on Saturday nights?

I am planning my bachelorette party for a Saturday night in March. We are going to awesome 80s prom at webster hall first, but then want to go to a club. My friends and I are in our late 20s. The only problem is that my sister, who is my maid of honor, is only 18. I am trying to figure out where we can go. Unfortunately I have been out of the club scene for a while and no longer have any contacts I can call in favors to. From what I am hearing it seems pretty grim, but maybe someone knows of something? Thanks!