What is wrong with my sister?

Severe PMS or a Personality Disorder? If you notice her moods becoming erratic and out of control for several days and then seem to improve, start writing down the date you notice the change. If there is a fairly regular monthly reoccurance of a mood swing in which this same irrational behaviour is exhibited, then it is probably extremee PMS. There are antidepressant that are taken approximately one week before and several days after a period to avoid these mood swings but she would need to see this as her problem. I've heard medication can do wonders for people whose hormones cause extreme anxiety and emotional outbursts.