12 yr old brother coming to stay with me (28F)(sister)

Hi all hope everyone’s doing well today. I have a couple of questions if anyone could help me out!!

Location: San Diego CA // Virginia Beach VA

My 12 yr old brother has been having some mental issues lately, he’s currently residing with my mother and sister in San Diego. He has recently ran away, after telling his therapist he wants to harm himself.

I’m his sister (28F) and I’m in the US Navy, (on shore duty orders) currently residing in Virginia. I told my mom I’d like to take my brother to come live with me, not forever but for a year or 2 max. And with that I’d like to sign him up for school while he is staying with me in VA.

So I read somewhere that I need power of attorney over my brother, so I can show the school that I’m not the actual parent but instead a legal guardian (older sister).

Basically I’m asking how would I go about this in terms of getting him into school for this year/ 2 years? And what documents would I need to show that state I am his guardian without potentially going to court over custody?

I just want to take him out of the situation he’s in (which is a lot deeper than I am saying here) and have him just focused on himself with no other familial distractions- just me and him, to show him that I love and care for him.

All help is good thank you in advance everyone.