I'd start with a good therapist. Bringing it up to your younger brother, who may or may not remember it, or who may not have the same feelings on it, could trigger things in him. Forgiving yourself doesn't require forgiveness from him.
His forgiveness doesn't erase it from your memory, either. Nothing will, but working with a therapist can help you work through your feelings in a healthy way, and perhaps, if your counselor feels it's appropriate, maybe they can facilitate a meeting with you and your siblings about it.
Start with yourself. Good luck.