I know there are few threads about this already, but my story differs from theirs, that's why I start a new one. If it's placed in the wrong forum I apologize.

So, I've got this great friend, we've only known each other for approximately a year but we're damn tight. I'm 18 and he's 17.

Few weeks ago we were at his place just chillin' and drinking some with a couple of guy friends. Anyway, his sister came home and started drinking with us, and after a couple of drinks she started hitting on me. At first I thought she was just playing around, but when she started giving me those looks etc I started to get her intent. It felt really awkward at first but then I gave in and started flirting back.
Worth saying is that it's his older sister, she's 25 and an easy HB10, she's one of the hottest girls (women if you like) I've seen real life.
So the evening was running out and it ended up with me making out with her in the kitchen, and he saw us.
The next day I apologized and he said it was cool, but I'd be damned if I lay with her. He said this with a great smile on his face and kinda laughing.

So what do I do if this happens again? Should I just refuse even though she's on me first? Or should I just go for it? I mean she's old enough to take care of herself and make her own decisions. And to be honest I don't know if I'd be able to refuse her with a little alcohol in the blood, she's that hot.

Any other information you'd like to know I'd gladly answer.
Thanks!

I would not do it out of principal. Face it the relationship probably wont last, this will leave a rift between you and your friend.