I am 23 years old, live in MA, and am seeking custody of my 17 year old sister who lives in in NH. My parents are well off but physically abusive and I have not lived with them since I was 17. My sister has been staying at my house and finishing up her senior year at High school in NH. I am a student and my sister will be a college student in the fall. I want support from my parents so that we can both go to college. Is my sister too old for me to claim her for support? I think until she is 24 as long as she is a student that I can claim her. I don't know where to start. Help please.

You are confusing IRS rules with parental custody.
You can claim your sister on your taxes if she is a full time student and under 24. She should claim herself instead; I'm not sure what would happen if both you and your parents claimed her. There are refunds available that go above what taxes if any were paid.

Your parents owe no support for any child once they turn 18 (unless there are provisions in a divorce for school, etc. past 18). If your sister moves in with you as soon as she finishes high school but is still 17, your parents won't have to support her either, unless she stays at home.

Thank you