This is a great situation for a guy to be in ( a guy having a girl to like him compared to a guy liking a girl but not knowing if she likes him or not). My suggestion is to just hang around both your friend and his sister. Go over to their house whenever you can and watch a movie or just to talk. If she really likes you, she will find a way to be in the same room as you and your friend. Just go slow and be respectful so your friend doesn't think all you want to do is screw his sister. Don't ask her out at first. Just be in the same room that she is in. Make small talk. Try to show EVERYONE, not just the girl, that you are a great guy. If you do this for a little while, the girl is going to start telling her brother what a great guy you are. Unless he is a jerk, he is going to want his sister to go out with someone nice, so he will tell you that his sister really likes you. The key is to just be patient. If you can do this, you will eventually be able to go out with her! Good luck.
I was there yesterday at his lil brothers bday party. I know how to act in such a situation, making a good impression, of course no sneaking around and stuff. The only thing I am concerned with in this situation is, do I want to date his sister? Cause I just met his sister a few months back. And we've all been hanging out more often and there's this tension between me and her. And we're all part of this groups of friends and we've been friends since highschool.
I was at his lil brothers bday party yesterday, talked to her a few times, even their mom. Her mom likes me too I guess (not in a sexual way of course lol, just really friendly). Her mom even made a comment about marrying a guy like me, which made me embarrased and her too I guess. Her dad is pretty cool as well, he acts like one of us haha. Last wednesday, we went paintballing and I shot her in the crotch. Which was so embarrasing...for me. She thought it was funny though.
I know how to deal with it. But thanks for the advice Nikki and Batwoman !

The only thing that got me thinking is, is this the right thing to do? Dating her? It's not like i'm a bad guy, even though I have the bad boy appearence but a good guy inside. I'm not gonna treat her bad or anything, so thats not the problem. Argh, I've never been in a situation where i'm dating a sister of one my best friends. She seems like a cool girl to be with. The brother is cool too, we hang out a lot, but I think he thinks i'm a player. We've gone out a few times, and some random girls approached me. So I think he THINKS i'm a player, cuz once he asked me to invite a few of my random flings because he thought it would be funny. I never actually flirted directly with a girl in front of him, only that some random girls start flirting with me. So yeah, the situation is actually quite complicated hehe.
I just need to think about it and right now I'm avoiding all the flirting with his sister. Well, semi-avoiding haha, its weird when you're checking each other out. I cath her doing it, and she sometimes catches me doing it.