I am so sorry that you are going through this. Have you heard of online therapy? Where you talk with professionals online?
Oh my. Yes, you need to insist on help. Cutting yourself to the point of using a razor to cut your wrists is very serious and very dangerous. Tell your sister, an aunt, the school counselor again, any other trusted adult. Sometimes we get crummy family and it sounds like your mom and dad may fall into this category. But use it to be stronger because this is temporary. You have your own life to build and will be out of that house before you know it. Stay strong, focus on school and try to find a social peer group that is supportive. I know this is hard if you are battling depression. And that you cut, this means you need immediate help. hugs