What should I do on a double-blind date?

Ok, so a friend of mine is having a date. His date asked if she could bring her sister around just to be safe. My friend asked if he could also bring me and she said yes, his point being that I should get his date's sister(he says she looks hot :p). What should I do? Should I go and try to impress her and stuff, should I go and just act platonic or should I not go at all because I'll only be a bother? And if I go, what should I do in this situation? Because we never met before and we'll have the two lovebirds right near us.