I suspect that the reason this is coming up now is that at age 15, you are thinking about your own sexuality and relationships.
Relationships with siblings and close relatives that cross boundaries like this are usually complicated. Most people who do some bad things aren’t entirely bad people. Many young women both love the person who hurt them and hate what the person did. It can be very confusing.
Do take into account that memory is a strange thing. It’s not perfect. It’s not like we take movies in our brains that are 100 percent accurate. As you pointed out, you can’t remember much. This is normal. Often when kids are overwhelmed or scared, they shut down the part of the brain where memories are stored.
The problem with trying to dig up exactly what happened is that the brain hates a vacuum. It often tries to fill in the blanks. It’s hard to know in situations like you describe which thoughts (“memories”) are accurate and which thoughts are an effort to make the story make sense.
If you think your brother will be honest with you and won’t hurt you, it might be helpful to talk with him. It may be that he has regrets as well. It may be that he will be relieved to have the chance to apologize to you. In that case, you may be able to repair your relationship and move on.
If, however, he’s the type to get defensive and angry and especially if there is any chance he would lash out at you, it’s better that you work with a counselor. Therapy can help you decide if it would be helpful to talk to him and, if so, if it would be helpful to have the protection of doing so with your therapist.
I hope you do whatever you need to do to put this situation to rest. You’ve carried guilt and regret around way long enough.
I wish you well.