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.
Hes 16. Thats how guys behave. Its not a generalisation because omg man must want sex, its omg hormones must have sex. Its hard wired into them, its how they act, its why guys start masturbating at 13-14 and why women don't till much later. We're different, its not a bad or good thing. Its just how it is.
I have never heard of any study indicating that women start masturbating much later. If anything, I've read a few saying that women start much earlier because our pleasure-causing bits work in childhood.
Maybe you're an individual but all of my female friends didn't start till much later. They started masturbating 18 years onwards, while it may not be typical, it is a common knowledge that society tells women that wanting sex is slutty, and that we shouldn't "do" those kind of things, because of this a lot of women have a perception that they're dirty or shouldn't be inclined to want to have sex.
I'm not talking about myself, I'm talking about studies, and I don't ask my friends because I assume that asking anyone in person would yield a high margin of error. I don't think young children are generally really exposed to the whole "only women are slutty if they want sex" thing yet. They don't understand that it's about sex. I specifically remember one article mentioning a 4 year old girl, though I don't remember if she was supposed to indicate the norm or a particularly young example. Kids reach that "this feels good I think I'll play with it" age before they really know what they're doing.
Many men dont understand the fact that when a girl likes them it doesnt mean that she wants to get in their pants right away.
Try to understand him as well. He is probably taking his time with you and taking it slow while you think things are going too fast. Set yourself a timeline and tell him about this. Like telling him that you dont want to go on the next level with the relationship until 1 month (for example) has passed. Tell him the reasons too.
Anyhu, he's probably realised you like him, you need to set some boundaries, its not hard to say no to certain things you aren't comfortable with, but make it known that if he goes past that mark you'll remove all kind of affection.
Tell him those people who can get hurt, why your sister would care I don't know, relationships/affection/sex is all a part of our psyche, so if she complains just chill. Its your life.
Your friend maybe a little uncomfortable, its weird, but not THAT weird. The issues would be if it failed, and then you or him got hurt. So I suggest explaining that to him, and sitting her down and explain how you feel, the position you're in and how you want her to be happy.
Crushes come and go, so if you think its too much to risk, just say no, if you think there is potential for YOU to be happy. Go for it. Also, there's no harm in saying you need him to cool down a little, he's probably got so many hormones making him so damn horny that he's trying to get as much as he can from you. Just say no.
I've made things sound so fast-moving, haven't I? Well, just so people know and I've failed to mention - I'm only 15 and he's 16, an he's soooo not like that. He's just, to sum him up, a nice, sweet, nerdy guy who's mega cute.