A., I am sorry for what you are going through with your mother-in-law. There are so many cases of undiagnosed mental illlness out there and your MIL seems to be one of them. I have an aunt who is just as sick, she makes life in our extended family uncomfortable -- has gone through long periods of not talking with various family members including her own children, her brother, etc. The very people most of us hold nearest and dearest. Our family has not fought with her. My mother taught us at a young age to just let her go on her tirades. It doesn't always make it easier. We are dealing with elder care of my 94 year old grandmother and the Aunt in question was a director of nursing at a nursing home. Auntie is furious that Grandma has put my mother and I in charge of her estate and medical decisions. Grandma has been in nursing home care for one month and EVERY member of the staff is familiar with my Aunt who lives two states away because of her actions while here for one week and her constant calls and threats to the nursing staff. They also understand who is in charge and WHY. My point in telling you this is that everyone in our family, everyone at the nursing home understands, if not at first, within a very short period of time the full situation and the same will be true for you IF you ALWAYS take the high road. Stop trying to talk to her about it, just be as nice as you can be -- force warmth into your voice when you say hello. You don't have to smile if she insults you, but you can just say, well I understand that you see it that way, and I'm sorry -- and walk away as soon as you are able. If you can hold your tounge in these situations and let life unfold as it will you will have more support from your husbands family because they will know that your family is not part of the problem.
My advice about the wedding would be to explain to your sil-to-be that you don't want an scene over your attendence to be the focus of the day. That although you would LOVE to promise you'll be there for EVERYTHING that you can't. Ask how you can help with advance planning, and try to attend the ceremony and the dinner and then duck out before people are free to move about. You can ask your bil to run interference if he's comfortable, just keep your mil away, and seat you at opposite sides of the room. Take your daughters if you can (most people will behave better around children). Tell your bil and soon-to-be sil that you want nothing more then to be there for them, and that you want nothing but a great relationship with them, and how important they are in your life -- but that you will simply not subject your family to bad treatment. Tell them that you don't want to put them in a position of making a choice between mom and your family ever, and that you want them to be there for the mom. This may mean seeing them separately, but always let "mom" have THE DAY. i.e. Christmas, Easter, etc., just plan your own special times together separately if you have to. Hopefully, your MIL will calm down in time, with my Aunt, her grudges seem to pass in about two years. When / if it blows over, don't think her temprament has changed -- you'll have to feel it out slowly, we've all learned with my Aunt to hold her near our heart -- but at arms length -- she's got some great qualities, but it's better to be safe and avoid all confrontation over all things.