Can you adopt or get custody of a sibling?

I’m 19 and my brother is 10. Our dad is deceased and our mom has a drug problem. For the past 3 years, I’ve been the primary caretaker for my brother. He’ll go back to our mom for awhile when she’s clean, but then she’ll start using again and he comes back to me. I want him to come live with me full time so he can get settled in the school in my area and stop going back and forth. Is there a way for me to get custody of him so that I can make his school and medical decisions, and so that our mother cannot just come and take him back whenever she wants? Can I get custody of him or am I eligible to adopt him? Which would be better? I live in San Jose, have my own place and job, and have no criminal record.