My Wife And I Might Need To File For Custody Of Her 13 Year Old Brother, But We Don't Know Where To Even Begin.

Hello, this is my first time posting. Location: Lubbock, Texas. We've been placed in a very weird and unfortunate predicament. Her 13 year old brother is in our care after a really weird set of circumstances. Their mom signed a paper granting us power of attorney for him in regards to the school he's going to. He's been living with us full time for almost 2 months now. We have him enrolled at a school down the street from us. Neither his biological father, nor his mother give us any financial support. We provide for him fully. The issue we need help with; over Thanksgiving break he spent the majority of the time with his mother. During this visit, she stated that she wanted to break our original agreement and have him move back in with her after she gets her own place at some point and time. Our issue; she is in no place to take care of him financially or any other means. When he moved in with us, my wife and I made the decision that we would do our best to hold onto him and help raise him until he at least graduates high school in 4 years. She wants to take him back against our original agreement and my wife and I want to make the steps to make the arrangement more permanent by either becoming his full time legal guardians or asking their mom to sign her rights over to us. Our questions; any ideas on resources we can use to help with the situation? I have a feeling we're going to need to talk to a family matters attorney, but we wanted to see if anyone has been in a similar situation and can offer some advice or if we even have grounds to try and gain custody of him. Any help would be appreciated.