And when you get over the shock.... ask yourself what is going to be worse for you: not saying anything to your best friend and having him find out down the line ( because he will find out eventually) that you knew and said nothing, or having him find out now, weeks before his wedding. Either way you're going to feel uncomfortable -- so there's no way around that. But it's also not your fault. You stumbled into this, and now you have choices to make.
My advice is to talk to your brother and your best friend's fiancee separately (and soon). Tell them that they owe it to your best friend -- and themselves -- to come clean and be honest. Explain that living this lie is going to eat away at them, and eventually, one of them will crack and tell your best friend -- and the other will seem like the bad guy in a bad situation. Encourage them to come clean to your best friend because that's really who should be talking to him. Tell them you can't promise them that you won't tell your best friend -- at least be honest to them about that. Explain that this isn't how you want to conduct your life, and since you've stumbled into this mess, you now have a secret you are not comfortable keeping and don't intend to keep.
Hopefully, they'll hear your voice of strength and reason. It's unfair for them to involve you in a lie. So let them know you won't be keeping their secret and therefore, you'll give them a week to come clean, but then you just aren't sure if, when or how you're going to talk to your best friend, but that that will be between you and him.