HELP,My future mother-in-law is trying to ruin my wedding!

Ok, so I am getting married in two months to the love of my life.There is only one problem, his mother. From the beginning she couldn't care less about our wedding. Now that the time is approaching she has even attepmted to talk my fiance out of us getting married stating that no marrige is going to last. She told him that because she had to deal with infidelity in her marrige, that he would as well. (already assuming that someone would be unfaithful!) Her biggest problem is that we have invited his step mother who his mother blames solely for her divorce. In over 15 years his mother hasn't had to speak or be in the same room with his stepmom. His mom says if the stepmom is going to be there that she is not coming. Not going to come to her own sons wedding? The stepmom is the one who has warmly welcomed me into the family and is also helping to pay for the wedding. Also we are having a victorian themed wedding. I have encouraged my guests to dress up in victorian style clothes. This is almost out of the question for his mother who states it is too much work for her to find something, she wont look right becasue she doesn't have the correct figure, and it is wrong of me to ask guests to dress up because of the economy. She seems to be looking for any excuse to be against the wedding. Yesterday, he told his mother that his fathers brother would be coming. She apperently hates him for something he said 20 years ago. Last night she calls and says that she thinks it would be better if she just doesn't come to the wedding. She says that all of this is making her so uncomfortable and stressed out and on top of that to have to dress up, that ITS NOT WORTH IT!!!!!!! So I wouldn't mind her not being there, but my fiance is crushed at his mothers uncaring and selfish attitude towards our union.She is taking all of the joy out of this for him and totally bumming him out.He told her that besides having children this is one of the last milestones of his life and she said "Well, I have been to all the others!" I cant believe her! So I need your advise. How do we deal with this without adding even more stress to this already stressful time. Do we write her off, or tell her how selfish she is being? Where do we go from here? HELP!!!!!!!!