She slept with my brother

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After RISE, my brother came to pick up me and my girlfriend. Usually, after every fete, she would stay over by me until she is ready to go home. On the way home, she was being a little flirtatious with my brother, I didn’t take it as anything; it must have been the rum. 

When I woke up I notice that she wasn’t in the room, so I went looking for her. Surprisingly, I found her naked in my brother’s bed. I haven’t said anything to her about it.           

The problem is, my brother’s girlfriend is a close friend of ours. She is currently in Barbados, but she will be back in a few days. Before she left she asked me to keep a close eye on my brother and report anything suspicious to her when she returns. This is a very big burden for me. I really don’t know what to do.