I'm going to out my brother-in-law's felony during the wedding
Why are you outing your brother in law so late?
I think it's fair that they know what kind of a person they let into their family.
I don't think you should wait to out him during the wedding as you will be the person that looks like a fool and also a wedding crasher. If you truly think this matters, you should tell people now before the wedding occurs. There's no reason for you to withhold this information for so long.
They are going to make a scene if I do that, I don't care if you don't think I should do it in the wedding.
You should first make sure that the bride is unaware of your brother in law's past. If they know and are fine with it, then you should leave their business to them. Either way, try to enjoy the wedding and be happy for your sister.