What you are experiencing are flashbacks. Any type of abuse you suffer, especially as a child, can cause PTSD. When you have PTSD, flashbacks of the event is common. It can be triggered by a smell, a place or even just a thought.
One of the key ways to address flashbacks is to identify what triggers them for you. Everyone is different so learning your own particular triggers helps. You may be able to control them such as when a certain place triggers your feelings, or you may not have any control over them in certain circumstances such as a sudden familiar smell or an unexpected memory. So learning what triggers your flashbacks can help you be more prepared to respond.
Grounding yourself can help. Flashbacks can cause you to feel you have moved away from present time into the past. Pulling yourself back through use of your senses can help. Try bringing something with you that you enjoy feeling, like a smooth stone. Keep it in your pocket and touch it next time you are reminded of what happened to you. You can also try something strong to smell, such as a sachet or a sample of your favorite perfume. When you experience a flashback, try to engage all of your senses to bring you back to the present.
When you experience a flashback, contact someone you trust and talk about it. By getting it out into the open, you give the memory less power over you. If you feel you can trust anyone in your group, talk to them. Also, try talking about how you feel about yourself to the group. Turning the abuse on you instead of on your brother, the perpetrator of the abuse, is harmful only to you. Learning to put the blame where it belongs takes time, but it can be done. Practice changing your thoughts about the abuse with the help of friends, trusted members of your group and also work on it yourself. Each time you think of blaming yourself, stop and think that it was not your fault. If a child came to you and told you she was being abused by her brother, would you blame her? You should treat the child in you the same way. She deserves to be cared for, not blamed for what happened.