Well the first thing is to stay calm, and from the info you have given, to understand that this is not your fault. It seems your future brother-in-law is having his own difficulties and is taking them out on you, and subliminally on your fianc�. He�s not a fool, he obviously knows how to hurt you both, a wedding type blackmail is a classic case, childish rants are another. He has nothing else to hold against you, I imagine.
I would take a step back, and encourage your fianc� to talk to his brother himself � not emails that would be impertinent. They need to resolve the issues between themselves, and then include you in the resolution. If the brother decides when talking to your fianc� that there is no turning back, then your fianc� needs to accept it make the decision not to invite him, rather than you.
He will only spoil the day if you let him.