What a difficult situation for both you and your dh to be in! Sounds like mom is trying to take care of your brother's situation, and I'd be willing to guess this isn't the first time - it probably even contributes to making him who he is. I totally agree with others who have suggested a flat out refusal to do the ceremony, for so many reasons. Someone who is unwilling to consider counseling, and feels as though only he alone understands his relationship with his fiance does not sound like someone who will work hard when the marriage goes through those inevitable tough spots. He even sounds like someone who would blame everything on his wife - from what you mentioned on your last post, he sounds emotionally taxing to say the least. I'd be concerned for his fiance in the long run.
And as for mom, maybe you could calmly explain your and your dh need to follow your instincts, and a desire to maintain your own boundaries, which she is crossing. I'm sure you don't want to hurt her, but what she has done is just unfair.
and good luck, whatever you decide!