Boards
It's not her fault for "putting herself out." If it were someone else who, as you said, didn't want to stop, then that would be rape and you'd be implying that it was her fault. She can wear whatever she likes and is comfortable in, and nobody should be able to tell her otherwise. Telling her that she needs to be more careful about what she wears isn't going to help at all.
And as for the original poster, I know what you're going through. I know how uncomfortable it can be around family members who maybe aren't so in control of their physical reactions to a pretty girl. That being said, I wouldn't know how to deal with it, since I'm often in similar situations and I still don't know how to react.
Personally, I feel like, if you have a close enough relationship with your brother, you could try to talk to him about it. I know that sounds scary, but if you said something like "hey, about that thing the other night... we're cool, right? No big deal?" Maybe if you said something about how you know that it was involuntary and you don't want it to make things uncomfortable between the two of you, that might help. Otherwise, I'd talk to maybe your mom or an older sister or aunt for advice. Even a teacher or guidance counselor could help you if you asked.
The bottom line is, boners happen. I'm sure it's not really as big of a deal as you think it is. If you're brother's 15, like you, I can almost guarantee that it's probably just hormones, puberty, messing with him. Even if you are his sister, the idea of having an actual girl on his lap could cause some unwanted feelings to arise. I think that, even if you didn't talk to him specifically about the event, if you just started talking to him like everything was normal again, you'd both forget about it and move on. I promise this won't ruin your relationship with him forever. 
I hoped I helped some. Good luck! 
