Looks like you will need to institute guardianship proceedings unless you are a biological parent, which is unclear from your post. This is probably not something you want to do on your own as there could be significant paperwork involved, court action, and further documentation requirements after guardianship is placed with you.
This should not be considered legal advice, but rather simple information. You should consult a lawyer to review your unique situation. No client-lawyer relationship exists until you sign something indicating there is.
I don't know that I would call what you have physical custody, it sounds more like temporary possession. I'm assuming you are not the father since you say you are married and the mother is a minor. In fact, it sounds like you are not biologically related to this child at all. That's not a small problem, it's a huge one. You can try this on your own if you like. I would say your chances of success would be very slim. You also run the risk of the Department of Human Services getting involved in this case. Additionally, a Guardian ad Litem will need to be appointed, so you will be paying for at least one lawyer anway. As far as the mother returning, custody of child is not permanent and can be revisited upon showing a material change in circumstances.
This reply is provided for information purposes only and does not represent legal advice or an attorney-client relationship.
You can file for custody, but you'll need the assistance of counsel. It appears you have a good understanding of the situation. The judge's foremost concern will be the best interest the child.
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