If you like him, have what I have recently learned is referred to these days as a DTR. That's a "Define The Relationship" talk, and it's when you bring up that topic of "are we dating? do we want to be boyfriend/girlfriend? is this just for fun?" etc.
At 15 and 16 you do not need to have sex in order to be boyfriend/girlfriend. You may hear a lot of older people say this or imply this, but it's because physicality in relationships works on a sort of sliding scale. In your younger teens, it's totally possible to have a relationship with no sex. In your 20s, it's more odd, and in your 30s it's odd to even date casually with no sex. But people tend to overlook this and assume that whatever level of physicality is the norm for their own age is the norm for everyone of every age. That's why you'll find parents/adults jumping to the conclusion that if you have a boyfriend or if you're dating, you must be having sex or be thinking about having sex.
That said, if/when you want to have sex it is perfectly within your right regardless of your age.
It's also okay if the two of you decide later in your relationship that YOU don't want to have sex but you're okay with getting him off by other means, such as using your hands and/or mouth (and it's okay to say hands but no mouth), or you're okay with him getting you off by non-penetrative means (research the clitoris), or you're okay with getting him off but you don't feel comfortable getting naked with him yet, or whatever other arrangement works for you even if it might seem to be an imbalance.
But this will probably be much later, and assuming things even go well enough to take you in that direction eventually.
If you have a crush on him and he likes you, I think you should go for it. Your friend might find it creepy that someone she knows so well finds her brother attractive, or that you might end up doing things like kissing and touching her brother. Even if he has had other girlfriends, it's easier for a sibling to distance themselves from that when it's a person they're not close friends with. You being her close friend might bring all your feelings for her brother a little too close into her own realm and it triggers that squicky anti-incest "ew this is gross I am uncomfortable" alarm.
Another thing to consider is that she might fear that she will lose her friend to her brother if you become closer with him than you are with her, which triggers sibling jealousy.
But I think she should just get over it, even if she might not know enough about herself to be able to do that yet. Make sure you stay close with her and don't disclose too many personal details about your relationship with/attraction to her brother, and maybe it will ease her fears and be easier for her to deal with.
Well I think that you should talk to your best friend directly (about her feelings about you hitting on her brother). It might be a little scary, but if you're true best friends, then you'll remain close even if you have a little disagreement.
I think you should also take a deep breath, and ask your crush if he wants to go out. I'm sure it will be scary, but you'll never know if you'll work out, unless you give it a try.
Thirdly, not to make you uncomfortable or anything, but if you're one of those kids who plans to have sex before adulthood (don't wait till you're married because that's just dumb), make sure you practice safe sex. I don't advocate 15 year old's having sex (unless you're doing it with yourself) but I know some of them do. So just be safe in that regard.
Last but not least: Be sure to have fun.
I'm always happy to chat via notes if you ever want to (I don't mind rambling). If you want to talk about nervousness or about how your crush asked you to dance or some other crap like that, I'm game.