Should I tell my best friend about his girlfriend and my brother??

I have a HUGE problem need advice ASAP!! I came home to see my brother (3 days ago) and seeing where he was at I went to his room to see if he was asleep and walked in on him and my best friend (been knowing since I was 2 years old) girl friend having sex. My mouth dropped now I am stuck in between a rock they begged me not to tell Franklin (my best friend) what was going on the past 3 months, 3 MONTHS this been going on, my brother also said if I was a real brother then I wouldn't tell Franklin and his girl friend begged me to the point where she had tears, telling me not to tell my best friend. Its just not right and I would feel wrong if I ratted on my brother cause we are real close brothers and I love him to death but I love Franklin just as much like a brother but I am caught in between a rock like I said and I need to know what to do. Should I tell my best friend or keep it from him and don't rat my brother and my best friends girlfriend out. Thank you guys P.s My best friend and his girlfriend are getting married in 3 weeks (March 30th) this is crazy.

My god that is alot to have on your shoulders. Firstly it is not your fault , you did nothing wrong. The only thing you can do is give them a chance to come forward themselves to their partners before the wedding. I think if both these individuals should not be saying I do, when really it is I don't know !!!!