I feel for you - it's hard enough just reading your story, and I cannot imagine how hard it must be to be not only abused by brothers, but also by parents!
I'll try to just stick to your questions:
1. First of all - you HAVE to go to police. This is no joke. If your brother was stealing your toys, well then you could resolve that with him. But sexual abuse is the worst form of physical abuse and doing it so early in one's life points to some serious troubles. If no one reacts to their behaviour they will with time only become worse, until one day someone gets seriously hurt. Therefore it is better that they learn as soon as possible how wrong their behaviour is.
2. Your parents are the worst kind of people. I am sorry for being so direct about this, but even though it is normal to love one's parents, that doesn't mean that you should pretend that everything was OK. You did tell them what was going on, and they did nothing to make sure it never happened again. It was easier for them to do nothing. And now you want to do the same and do nothing again? Then you'd be the same as them! So the answer is - NO! You don't want to be like your parents, you'll do the right thing and report the whole thing to everyone that should know about it.
If you don't report it, the story will get out of hand and everyone will talk about you as the girl who likes to have sex with her brothers. Would you rather have that?
Everyone who has any feelings for their abused child would react HARSHLY to your situation. Calling police would be one of the things they would have to do, at least because it is required by law (to report all criminal activity).
If your parents were any good, all this would've never happened. This way, police is necessary. It would be interesting to see what else they find out...
But THE MOST IMPORTANT THING: when you're attacked, when you are forced to protect yourself as you are now - DO NOT TAKE INTO CONSIDERATION IF THOSE RESPONSIBLE WILL BE HURT WHEN YOU DEFEND YOURSELF! I mean, if someone tried to steal your money, and you ran away - would you wonder if you should maybe come back to him so that his feelings are not hurt? Should a raped person appologize to the attacker and ask him if he wants some more? Do you understand what I mean?
I don't think that your brother or your parents really care about how you feel about all this. If they did, they would not hurt you. So why would you care if they get hurt because of what they did!?
Abused people through abuse start believing that they are worthless as individuals. Therefore, they don't feel that they deserve protection. And in your case, calling police is actually asking for protection. And even if police don't believe you, you have to continue the fight. Find someone in social services, someone at school, someone who you trust and who will guide you through all this. You HAVE to become a fighter not a victim.
You know that Oprah Winfrey was also raped as a teenager? Do you think she'd get anywhere if she just felt sorry for herself and never told anyone about it?
That's what you need to do: fight when you're attacked and fight until you WIN.
Believe me - bullies of all kinds will only attack those who don't fight back. Now is your turn to fight back, and you'll see how everyone will respect you more afterwards. If you don't fight back, you will be attacked again and again and again... the evil won't stop just by itself - it never did.
3. Why all this happened to you?
It's like a show I saw on Discovery channel long time ago about the most beautiful girl in a highschool who got raped and murdered. In the end when they found the guy who did it, do you know what he said why he did it? He said - "Because she was so beautiful, popular, and smart - all the things I never was". He killed her because he was jealous.
Your brothers are also jealous of your qualities. We all think that bad things happen only to bad people, and they do. But good people are much more likely to be a target as all those bad people are going to be jealous of them. It's hard to understand this, but look at everyone around you and you'll see that whenever someone does something well, jealous types start putting them down and attacking them.
So, because you are abviously smart and a good person, you need to learn to take care of yourself, just like Oprah Winfrey did. Killing yourself is the stupidest thing you could do - it wouldn't solve anything, and some time in the future your life is going to have some wonderful moments that you'd never see if you stopped living now. I now that right now you feel terrible, but that is because you are taking on yourself not only the burden of being raped, but your family's burden too. Frankly they suck, and you should make sure that they all get some sort of punishment - let them deal with their own crap they created. Then you'll see how you will start appreciating life much more.
Make sure you always draw the line of responsibility. If you rape someone - then that's your responsibility and you pay for it. If someone rapes you and someone else doesn't protect you? Then that's their responsibility and whatever comes out of it is still their responsibility. None of your business.
I can understand that you want to stick by your family. I would want to do the same. But I am a bit older then you and have learned that family is NOT necesarily your father, mother and so on. Family are first of all PEOPLE WHO LOVE YOU! And love means support, protection, undestanding, and so on. Your parents are none of those. Therefore they are not really your family, except biologically. I mean, what if your brothers killed someone - would you help them out regardless of the pain they caused to someone? If you did help them out - you'd be an acomplice.
There is a line as to when our family should be protected and when we need to turn away from them. As soon as they stop protecting us and start hurting us, they are not our family but our enemies. The sooner you realize that, the better.
Did you know that most murders, rapes and other violent crimes happen right in the family?
So be brave, don't tell your family about what you will do, and do go to the Police. To make sure no one thinks you wanted to have sex with your brother, to make sure everyone knows that if they try to hurt you they will end up in trouble, and to be able to walk out of this feeling much better about yourself.
As for your dark thoughts, and misery and so on, I can assure you that as soon as you leave this family you'll start feeling better. However, you will need a bit of counceling/therapy. From reading your text I can see that you are a brave and exceptionaly smart girl, therefore you'll do well in life. You'll have a nice job, you'll probably do something you like, and some day you'll mary some nice guy and have a bunch of cute kids and go all kinds of places and see most beuatiful sunsets on the beach... all those beautiful things that make life worth living.
Love hugs and I will pray that you fing strength to deal with all these troubles that others have created for you.