<human>: Can you succinctly summarize the text in 1-3 sentences?

This guy and I met in college and hit it off really well as friends. I developed feelings for him but never told him. Eight years have passed and he confesses that he has feelings for me, wants to be with me and wants to be serious about it. He told me that he really feels like we could have an amazing relationship and pictures a future with us. I'm so thrilled and want the same… But a few years ago I made the stupid mistake of sleeping with his younger brother (26M). He and I were also good friends, and though I never had romantic feelings for him, I slept with him after we got drunk together (I had recently broken up with a boyfriend, he had a serious fight with his wife, and things just happened...). We promised we would never tell a soul, but I'm not feeling too great about keeping this a secret from the brother I am now dating. I cannot tell at this point if I can get over this or really feel like I should say something… I guess I'm looking for some insight on what I can expect if I do say something, or keep this to myself. I don't know if this makes a difference, but I have not seen or talked to the younger brother since it happened. He and his wife moved away so we haven't been a part of each other's lives for awhile. Thoughts?
<bot>: A few years ago I slept with the brother of my long-term friend/ now boyfriend. I don't know if I should inform the boyfriend or accept it as a mistake of the past and let it be.