Abuse repeating itself...advice please

I am currently 20 years old but I grew up with child abuse mainly from my step mother, but also my step father once I was in college and still living with them.
I was convinced my mother must have bipolar disorder or some other chemical imbalance, because my sister and I would have to walk on eggshells constantly to try not to tip her off. When it happened, she would just snap.
It started when I was in 5th-6th grade, but finally when I was 17, I had had enough. My parents were good at placing the blame on me and I believed them. After a night when my hands were all cut up from my mother breaking her nails slapping me, tearing out my hair, and slamming my face into the ground, I finally called the police from a house I was babysitting at. They came, but my mom convinced them that I had attacked her and done the damage to myself to try to blackmail her. They believed her, took my two sisters out of the house, and left me alone for the night in the house with her. She came to me and asked why I would act like she had done that to me. She is great at being fake and pulling things like that off.
After the police call, however, she would not touch me or my sister again (we were both not biologically hers, and she had another daughter that was hers that she wouldn't touch).
After a move, my dad began to be abusive with me when I turned 18 and started college, I believe because he felt like he was losing control of me. I soon left their house, and we have had a rough relationship ever since, because they didn't want me to leave.
I thought everthing was ok with my sister, who is now 13. But last night I talked to her and found out that my mother is abusing her again. She even went on to say that my mom poured a bowl of cereal on her, broke the bowl at her feet, then told her to clean it up. When my sister grabbed the windex, my mom asked what the hell she thought she was doing and sprayed windex all in her face, hair and mouth.
She also told me that they yell at her for flinching when they start to get out of control with her, and will hit her everytime she flinches until she stops flinching. She now flinches everytime there is a quick motion around her, and she gets laughed at and teased by friends.
It breaks my heart to hear my sister going through exactly what I did. My sister says she is miserable, wants to end her life or run away. She even joined the band class because she wanted to take the band trip and then dissapear in a crowd while they were away.
I want to do something to help her and would even volunteer to take her myself. I am afraid to get the state involved because I don't want her in a foster home. I am not close to our distant relatives, and just don't know who to turn to or talk to about this.
She lives in the state of Florida. I live in New England.
What should I do?

How terriffing for you and your sister.

You stepmom does not have the right to treat an human being. I would talk to your sister privately--If she is reported your sister is young and if there is no "good" family, she may be taken away. Or your Stepmom might have to leave the household. I dont know what the laws are there.

I refused to live with my mom and stepervert. And my dad and my stepmom did not want me, so I went to the county group home.
I learn something to, In the normal world humans don't get beat on at all almost.

All those times she ripped out my hair, grab me with her long nails and leave purple half moons and blood red streaks that shirts dont hide.

All those times and I never hit her back, I wound up learn to steel myself from her blows so I would just look at her when she beat me, I look at her and give NO REACTION. freak huh?

You could get a game plan with you sister... and go for getting her out of that toxic enviroment. No one deserves to live in that.

all this is JMHO of course. I and stand it is a really bad situation, and the choices made are very hard to make.

All my Best!

You are not a freak for that. I used to do the same thing; I would give my mom a cold blank stare with no emotion, but it used to piss her off even more. I guess you just learn to shut yourself off when it gets that bad.

I would love to just take my sister and leave, but obviously she is legally theirs, and she could call the police for kidnapping.

I am thinking of asking a friend who is a cop here for some advice, but my heart is still fluttering and I don't know what to do. Whatever it is, I want to get her out of there quickly.

oh yeah, it would piss my mom off too.

She was in my face to the point she almost bit my face!

more that one time. She raged. Would you believe her and I are ok with each other? yep!

You should decide what is the best and quickest way. I agree.
Ask the cop, that is what I would do.

All my best!!

Maybe you could call the local Child Protective Services and tell the authorities you will take her yourself - she's got to get out of there - and you will both need to heal yourselves from all the horrible things that were done to you or you will continue to relive the abuse for the rest of your lives.