Hi, I'd like to help you with your question.
I am sorry this is happening to you. It is difficult to deal with someone who is trying to make your happy time difficult.
There are some ways you can handle this.
One, sit down with your finace and talk about options in how to deal with your future mother in law's behavior. Your finace knows her best and give good ideas on how to handle the situation.
Your fiance should talk with his mother about her behavior. You should stay out of it. If you try to deal with her, it will only make you a more attractive target for her and he will appear blameless.
Your future mother in law may have several reason for her behavior. She could be jealous, or sees you as replacing her in her son's life, or she is so close to her son that she feels he is being lost to her. Whatever the reasons, she cannot be allowed to interfere with your engagement, marriage or the raising of any children you may have. You are the wife and therefore the most important person to your husband. Your mother in law is not.
Whenever your mother in law does something negative, try to smile and let it go. It reflects badly on her and leaves you blameless. This goes for discussing her with others as well. Whatever you do, do not say anything negative about her. If someone brings her behavior up to you, simply respond neutrally, saying that you don't know what is going on, or something to that effect.
Make sure you and your fiance are discussing this on a regular basis and that he is supporting you. If this is a problem, seek out counseling. Although he may feel pulled and confused by her behavior, he needs to put you first.
There is a book that may help you called The Daughter-In-Law's Survival Guide: Everything you need to know about relating to your mother-in-law (Women Talk About) by Eden Unger Bowditch and Aviva Samet Psy.D. You can find it on Amazon.com or your local library may have it available.
I hope this helped you,