My sister is psycotic help!?

so I have on countless occations helped my sister with her home because she can NOT clean it. I have ALWAYS been nice to her, and NEVER EVER said anything mean to her. SHe has countless times called me names, and been extreamly rude to me. She is 36, has two kids another on the way, a husband and a house! I am 21, going to school, and live at home. My Niece (her older daughter) wanted to throw a suprise shower for her, and so I was told to help by my mother. so I didn't know any of her friends and got them off face book. We got all the invitations, and then my niece (who had previously like 2 months ago asked my sisters best friend for help with the guest list) got yelled at by my sisters bestfriend, and told they had set up one on their own, but we had already contacted all these ppl. So mother asked my sister to talk to me, and tell me that she appreciated me I did NOT ask my mother to do this. so yesterday my sister texted me that I should keep mother out of this except she has put mother in the moiddle all of the time! I texted her that she was being hypocritical, and then I came home,and she was there. SHE NEVER says hi to me so I didn't say anything, and went up stairs after talking with mother. THEN I hear her SCREAMING at mother saying I never do anything for her, and that I have a problem. she also texted me this. then mother told her to leave, and she went in the car and cried, so mother "felt bad" and tried to make her feel better. Next day I posted that it's disgraceful to scream at your mother who is always doing things for you. THEN she started attacking me on face book, and yahoo, and I didn't reply, she has been doing it all day and won't stop... idk what to do 'caz I don't have anything realy to say to her, and she just screams at everyone, so I don't wnat to talk to her.... what would you do?