Mother In Law to Wear Black or Not Wear Black?

I just uploaded a center piece question a few seconds ago but I just thought of something. My wedding is going to be a black and white theme but in theory when the mother in law wears black that means that she is unhappy with the wedding. Me and her have had a history of her being terribly mean to me and saying little snide remarks. Me and my fiance was engaged once before and when we told her, the only response she had was when was the wedding. She did not care and was very hostile. Over the years we have become a little closer but I am not sure if we are much further along now in our relationship than we were before. Should we include in her invitation that she is to wear white or another pastel color on that day? I don't mind if others wear black but I am not too keen on her wearing it. How should I approach that with her?