There isn't anything to do. Mother in law has every right to deny her illness and refuse meds. If she is an unpleasant nightmare to be around, choose not to be around her. If the fiance cannot choose to cut back contact with mother, perhaps rethink the relationship. His unhealthy choice could cause unneeded stress and all sorts of problems. My advice to live your life means to choose who you want in it.

All anyone can do is live her own life. Other people do the same and choices about others? Who we want to contact, how often and obviously, procreation. You can't change people. People can change themselves but, that isn't your responsibility.