Your fiancé is not so amazing if in response to his mother's outrageous, sickening request he didn't immediately say to her, "Mom, Elise's dad is a great person. That he has overcome a terrible trauma makes me admire him even more. You need to permanently drop this. He'll not only be there, he'll walk her down the aisle, and I don't want to hear another negative word about him." Instead, he has weaseled around, and presumably didn't tell his mother not to make her despicable request to you—he surely knew what she was up to and didn't even have the courage to warn you. Instead of responding to his mother, you need to talk this through with you fiancé. He should be the one to respond to his mother about this, and it's not too late for him to make clear she is totally out of line. How he handles this will tell you if he's worthy of becoming a member of your family. And I hope you tell your parents that if they are not both at the wedding and treated as guests of honor, you won't be there, either.