living with an ex-abusive sister

context: my sister has been at uni for three years so the only time i have lived with her has been short term holidays and half terms. before uni, she was verbally, emotionally and physically abusive - my life was hell then once she left things improved significantly. now she is moving back home, refusing to pay for rent,food ect. she doesnt wash up or clear up after herself ever and i end up having to do it or things would go to chaos. she is rude, arrogant and expecte everything to be handed to her on a silver platter. she claims to be broke when she spends massess on insignificant materialistic things too, saying she cannot pay rent. my parents dont do anything and feel like they cannot control her. im supposed to go to university in a month but results day is next week and everything is so uncertain. i dont think i can live with her for a month. i think my mental health is going to significantly decline as it was before, and i can distract myself with work and running ect but its not enough.