It's OK to forgive him. It's OK to be OK, if you really are OK.
But it is NOT Ok to NOT prosecute him.
Because he's going to go out and do it to somebody else. And somebody ELSE. And eventually he becomes a serial rapist and is either in jail at a later time or dead at the hands of the police or a would-be victim.
Even if YOU don't need help, HE does. If you love him you're going to get him help.
First and foremost: this is an opinion. I am neither a psychiatrist, nor a lawyer – not even American. Please read my written thoughts with a grain of salt, because it is full of assumptions, which may be totally false. Please remember this, and by reading other responses too, draw your own conclusion.
I don't know what he did to either you or your sister, but I would bring your family's attention to the possibility that your brother would be at the absolute bottom of the prison hierarchy: He would be probably treated the same way as pedophile rapists. It might become a death sentence for him, especially considering that his family has most likely disowned him, so he might have no life to return to, unless he already has his own elsewhere, of course.
However, you must also see that your brother committed treason and abuse of power against his own family, so he does deserve a harsh punishment. Sometimes love is best expressed through disciplining the one you care for, but punishment is only good if it brings about personality correction.
Oh, and one more thing: be honest about this. Don't try to protect him with petty lies, it will only discredit you, and might even cause more trouble for him. Tell your honest feelings along with the truth instead, and always ask yourself what part of it made you feel the way you do: For example, it might feel slightly complimenting to be molested, because it's an admiration of your feminine qualities; however, it may be overwhelmingly humiliating to be used selfishly and without any respect, concern or remorse.
When it's someone you know and love, sometimes it does happen that you don't want anything bad to happen. People react to a rape differently. Most people often won't care who it was. Especially in this kind of situation, I could imagine your parents being extremely upset one of their own children would rape or molest someone, especially another child of theirs.
First of all - I think it's strong of your family (sister and mother) to press charges against your brother. It must be hard to do that to a member of the family even if they done something much worse to you.
As for you. Since it is someone from your family, whom you been close to, its not overly strange that you feel shattered about this. I would suggest talking to someone, a therapist or such, just to straighten out your feelings and process everything that happened (both with the rape and everything that is happening now). And don't beat yourself down over feeling "wrong", because you can't help what you feel.
You are wrong, but also not abnormal. Your behaviour and reasoning is not uncommon in victims of abuse and it's unreasonable to assume that simply knowing it's wrong will change your opinion in the slightest. I'm surprised you're not all in therapy though I don't know how long ago it was revealed, until that time the least you can do is not interfere with your sisters opinion or testimony. Depending on how compassionate your parents are I would probably talk to them privately, if they're not going to entertain the idea that you're going through it in a different way then save it for a therapist.
I have been through a similar situation, but with an uncle, and it was not rape but physical and emotional abuse. At some point, he tried to burn his own daughter alive. So, sit down and think about this situation carefully.
Forgiveness heals only the forgiver. It does not fix the problem.
It would not have gotten to this point if your brother did not do those things.
I can't even begin to help you, because I have no experience or knowledge worthy of your problem. I'm just very sorry this happened to all of you.
I will say this, however: Do not try to dissuade your sister from pressing charges! Do not tell her she's being mean or wrong. Just don't. You might feel like it's sad for your brother, but your sister is a victim, and she is right to seek justice for herself. Please don't make her feel bad about it.
Therapy is a good thing for all involved. You, your mom, and sister should all go together as well as individually. It can help you get over the hump, no pun intended. No one knows how you feel unless they have been through the same thing. I'm sure as a first offense your brother will not go to prison for long or maybe not even at all depending on what he did as molesting can have many definitions.