There is nothing wrong with you. There is something very wrong with your brothers. I don’t know where they got the idea that it is okay to molest their little sister. I have to wonder what they have experienced that would lead them to do such a terrible thing.
The reason you didn’t say anything in either situation is called the “normalcy bias”. When a person is victimized or witnesses a disaster or crime, the brain may refuse to let it in. The brain protects the person by trying to make the situation feel “normal” even though it’s not. It takes a while for the person to process what is happening and to realize that something is very, very wrong. It’s a common response to a shocking or traumatic event. It’s actually a way that people can cope with something that is emotionally overwhelming.
You didn’t tell your brothers to stop because their behavior was so shocking and outside your normal expectations of how they should treat you that you couldn’t process it. You have nothing to be ashamed of. You can’t expect yourself to be prepared for something that is so unexpected and so wrong.
Yes. You should tell your mother. You need her protection. You might also benefit from talking this out with a therapist in order to feel better. Your mother also needs to take steps to find out what is at the root of the problem. Your brothers are both troubled and troubling boys to be doing this kind of thing to you. They need help before they go too far with someone and commit an even more heinous crime.
I’m very, very sorry you have carried any guilt for this. Really. It’s not your fault. Please do talk to your mom and think about talking with a therapist to help you get past this.
I wish you well.