How can I get into a 18+ club being 17?

I want to go to a club thats 18+ and im 17 and my older cousin is 18 and she is going and she said that the bouncer might be able to let me in if its her friend, but her sister who is 15 is getting in (making a fake ID and getting a hookup). I started asking my younger cousin, 15, for more info and she asked if I was 17 to be sure, and said if I have ID and I said yeah but they don't say 18 and she said that we just flash the cards and she can write Xs on my hands. (sounds confusing, read carefully) So what should I do? Do they really just flash the cards or do they thoroughly check em? Please help me! Oh and its the Dayglow event at Club Aqua on March 5th.