What should I do with my evil mother-in-law to be?

You have to get your fiance on the same page with you on this or you will be, probably forever, fighting an uphill battle. You going off on her likely won't do anything but validate, in her mind, the reasons she's against you marrying your fiance. You don't say anything about why your MIL to be is completely against your impending marriage. If it's a simple case of her having a hard time with the fact that you will be the first woman in his life from now on, you might try to extend an olive branch and spend more time with her along with your fiance. If it's something else and/or if you just don't feel compelled to try to have, or improve, the relationship with her, you'll need to get your fiance to acknowledge his mother's attempts to sabotage your relationship and he will need to tell her to stop. Any man who is not willing to stand up to his mother on behalf of his wife had better prepare himself for a lifetime of awkward holidays and strained family ties. Not fun.