My f21 bf m27 touched my younger sister inappropriately. How should this be handled?

My little sister was drinking over the weekend and told my sister that he drunkenly touched her on two occasions starting a month ago. The first time I was sleeping and they were both next to me, she said that he touched her for a second and then jumped back saying that he thought she was me. The second time he was on the couch with her while I was not there. She said that he did it again on her thigh very quickly and moved back again. She also said that she thought she may have been dreaming as she has very vivid dreams sometimes. I tried to talk to her again about it and she immediately became upset and told me that it probably didn’t even happen and that I shouldn’t break up with him over this. She said if it did that it was not intentional and she wasn’t worried when it happened. She said that was also the reason she did not tell me initially because it didn’t seem like a big deal. She is saying it’s a dream but I’m going to treat it as reality. I am wondering if there is any way we could stay together. I know it was an accident and he was very confused that night, I had woken up later both times and he was nearly incoherent. I believe if the drinking stops things could work. However, this might just be wishful thinking.

We broke up a day ago, I am wondering if this can ever be reparable in the future?

On one hand I feel that it could’ve been an accident and with no more drinking things would be okay. But on the other, it’s the fact that it happened at all. That is something that should’ve never happened.