Please, I don't want to undermine your experience, however, sometimes our perception of events can change as we mature.
Is it possible that due to your age's at the time, there was no intention to violate you & as a young person he hadn't developed self control etc? or perhaps he wasn't aware how forceful he was acting out!
As young teenagers, they are experiencing new sexual urges and new interactions but with added sexual inexperience or lack of knowledge of right or wrong! (Years ago boys were taught that if a girl said no.. "They were playing hard to get"!! (Please know that I don't agree with this, however I remember those sayings, while growing up & maybe those words MAY of stuck with him! & he felt pressured!!) (I know it's not right!! But I'm trying to view a different perspective to help understand how the situation may have arose without intention)
Is it possible that as he has naturally evolved he is now aware of his actions on that occasion, where he looks back on it & realises how inappropriate he was acting out on them? (Maybe he has tormented himself of those actions & feels deeply guilty & full of shame)
How to move forward... initially I would
Look to see how this person has grown into adulthood... is he a family man, kind & thoughtful or is he an a**hole??
If he has grown & matured in a healthy way, then I would work on trying to process the situation through innocence of adolescence (we have all done things differently without key knowledge) & then talk through your feelings with a therapist) some work placements will pay for some sessions) which maybe helpful to help you process the impact & your trauma responses)
I would also, find a quiet moment to discuss those events with him directly & allow each other to address the elephant in the room (this could be therapeutic for you both, as he may well be holding on to negative thought's & emotions because of his actions that does not marry up to what his adult core values are now) he may feel elements of guilt & shame! which in turn may help heal your hurt & resentment.
If he has not grown or developed and remains an a**hole then I would confide with your brother and allow him to tell his friend, he knows what happened & he is no longer welcome to join in your family reunions.
Just to note, I am speaking with experience, as a child and then as a teenager, I had been subjected to various SA by different people (including those who should have protected me!)
I then had a 5 yr abusive relationship 13-18yrs old, where I was subjected to daily emotional & physical abuse including rape! (At that time, I did not know it was deemed as rape as my limited knowledge was defined by a dark alley with a stranger!) I had no idea that you can be raped in a relationship, however as I have aged & been educated, I now know the difference, yet I can also look back & acknowledge that my then BF had no understanding that he was actually being abusive. Therefore, I hold no ill against him, I can let go of that side of hurt & resentment and I have focused on how I feel/felt and find ways to manage my trauma responses.
I hope my post doesn't cause you further distress by sounding like I am advocating for your brother's friend! & dismissing your traumatic experience, I can honestly say that is not my intention.
However, I wanted to bring a different perspective to the way we think, as our brains are stronger wired & they can influence our feelings!
A sense of understanding, helped me to develop a sense of forgiveness, which allowed me to let go & heal & move forward.
Sending you my best wishes 🫶🏼