As good as you have been to your nephew, he is now running afoul of the law. He could very well end up in juvenile hall without some strong discipline and guidance. Boys and Girls Club, churches, police Explorers all come to mind as organizations that help young people who have the problems your nephew has. Both of you could meet some new friends and he could possibly be turned around before anything more serious happens.
Your mom, I am sorry to be blunt, is a criminal. She knows it, too, and she knows she can take advantage of you. Seriously consider reporting her to tax authorities (the IRS and your state department of Revenue if your state collects state taxes on income). I know you don't want to report your mother, but she has really been taking serious advantage of you and has been breaking the law.
You need counseling. There is likely some free or low-cost counseling via your County or places like Catholic charities, Jewish family services, or other religious organizations, no matter what religion you personally are. These groups get federal funds, I think, so they have to provide to anyone who asks, not just to adherents of a particular religion. Your local universities might have a school of Social Work and might provide counseling through this school.
You are understandably angry and upset because you have allowed your family to take serious advantage of you. I hope you get some assistance with life in general and regarding school. Do you want to become an RN?
best wishes to you and your family. Take action tomorrow by going to church and meeting the pastor, elders, women's ministry leaders and youth leaders. Do not delay another second.