Okay, so, my sister is always ‘on edge’ in the way that she can be totally happy and then she’ll just suddenly snap and throw a tantrum (she’s a 20 year old with a one month old). She does this even when what you’ve said is a general question such as “What time’s dinner?” .. she just seems to think it must be an insult. She has also done many ‘attention seeking’ stunts such as pretending to faint and pretending to overdose and pretending to be raped.. I’m afraid one day one of these things may onr day come true and nobidy believes her. You can also not mention to her anything about getting help or else she gets angry. I’m really scared and worried and she’s kind of splitting the family apart but she doesn’t seem to even notice.. All she really notices is herself
Your sister needs to be psychologically evaluated. If you cannot tell her that yourself, get family members and friends and stage an intervention showing concern for her. If her daughter is in harms way call Child Protective Services to help give her a wake up call to realize that she needs help.
It sounds as if she has a mental condition where her mind believes everyone is out to get her. There really isn't much you can do except try to nudge her to seek professional help. If you can, try to avoid her and maybe not speak to her unless you have to.