How can i help my sister ?

My sister is tired of being abused and threatened. When she doesn't water her cats our mom threatens her by taking water away from her she's sick of it she has mental disabilities so she can't move out on her own she's starting to feel abused and threatened also she's upset and worried. Even though she's 28 she has to have an adult with her at all times because of her disability she can't figure stuff out on her own and has a hard time understanding things also doesn't pay attention half the time. She wants to move in with me. How can I help her ?