A very hard situation to handle, I will say. I've had similar things happen, and learned a lot from the mistakes and actions. This could be because there was a lack of sex in the relationship or lack of interest in both parties. Most relationships that involve affairs are lead up from lack of a sex life in the relationship, or lack of sexual interest in the other partner.
If there was a lack of interest, that's not your fault. If she found a lack of interest in you then you are just not good enough for her. It's most likely hard to believe right now but it's true. If you think about it, anybody who thinks they are better than anyone else is in reality on a lower level than the person receiving the insult.
I'm not saying this is the issue here, but it is a possibility with the information given. There would be no other reason for somebody to cheat on a person with no reason given unless it's a nymph. If she is a sex addict then who she does it with doesn't matter to her. It is possible for sexually active girls to become sex addicted regardless of the age. It's uncommon for teenagers to get to that point, but there's rare cases of this happening.
Though if it's been happening for months, then maybe she wanted to break up with you but didn't have the heart to do so which was totally wrong. I'm not saying this to be mean, but I'm just trying to give you open options on why this could have happened.
Fact of the matter is she shouldn't have cheated on you in the first place and just talk to you about it instead of going off and having sex with your younger brother. Violence is not the correct approach here, but simply talking to your brother as you did but more extended could answer a lot of questions you may have. Ask him did she say anything to him that involved your relationship with her. If she was saying something behind your back, you will now know the reason why.
I'd say settle down a bit (even though it's been roughly 8 or so days) and think everything over. It can take weeks, months, even a year to get everything straight. There's no reason to rush this and hints why I'm giving somewhat late advice (just saw your post)
I've had similar experiences and I'll be willing to be open with you to help you get through this if you'd like. My MSN is open if you need to talk about anything. =)
