Custody of sibling (sorry I didn’t know where to post)

So my mom has four children. She only has one left in her care but my mom is the worst ever. The two middle children basically raised everyone while my mom was never around whether she was working, running after her boyfriend, or sleeping. Well I was one of the middle children and out of my other two adult siblings I am the only one with my shit together. My moms health has been declining and I have shown no pitty or even care because we don’t nor ever had a relationship for me to care. Well her husband (the boyfriend she used to run around after) was recently locked up. Also my mom has been homeless for a few months now. I have offered my place but she won’t love hours away due to her new job. Well she contacted me yesterday about giving me custody of my sibling due to death threats from my step dads actions for being Locked up. Of course I said yes I love my sibling too death but lately I have been thinking. My sibling is 16 and my mom has never given any of us any stability and I want to change that. Even though this is temporary custody. What can I do to gain permanent custody because I know my husband and I can provide and give my sibling full stability. My sibling even knows we can. I just want what’s best for my sibling and with everything going on I just want to keep my sibling and help her because she is only a sophomore and a year behind. I’m a stay at home wife and I know I have the time and energy to help her and become involved in her school/life. What do you think? Anyone know where do I begin to gain permanent custody?