This has a lot to do with mood swings they experience. My mom is the same way. The doctor recommended Zoloft to help with the mood swings but I am trying to determine ways to get her off of meds due to the awful side effects. The neurologist recommended avoiding certain situational triggers, rephrasing how we talk to her, help her think the ideas we come up with to help her are her own, keep her mentally stimulated with games and music, and try to get her to help do activities around the house so she feels helpful and less argumentative. I hope this helps you and your sister start to explore some ways around the mean attitude and mood swings.
