I am a 25-year-old woman seeking custody of my 16-year-old brother who is out of control in my mother's care. We are technically half siblings, but I raised him. His biological father is my stepdad, who adopted me when I was 13. My parents both worked a lot and so I was left to care for my brother since I was 12 in MD. Now, a short 13 years later, after our dad died, my brother is out of control. My mom is never home and has pretty much left him to his own devices. I currently reside in SC and want to have custody of him and allow him to finish high school with me. What do I have to do to make that happen?

You can take action if you believe that the mother is not taking care of her son properly .
It is possible for you to seek Guardianship of the boy first. This would involve going into Probate Court. Then, if and when guardianship is obtained, it may be easier for you to file for child custody in your role as Guardian. In effect that Guardian might be considered to have standing.
But... you would need to approach the police or some child protective services agency in order to register a complaint, rather than filing an action in court on your own.
There is a strong presumption in favor of custody of children being with their own natural parents, as compared to being with the third party.
Therefore, the courts in child custody disputes that involve third parties will, absent strong circumstances, keep the children or return the children to the custody of their own parents.
On the other hand, the court will be persuaded to give custody to you when looking at the circumstances involved it is presented with clear and convincing evidence that it is in the child’s best interests to be given to you.
There is "Standard of proof".Some of its circumstances may be as follows:
-the home of the mother for some reason is not fit,
-the mother is not fit
-the child is old enough, such as being a teenager, and does not wish to be with this mother.

GV70 sounds like my friend who's in law school. Their right get a lawyer cause there may be some legal confusesion