Dad pays mom child support for this niece and three siblings. Niece has been with us since August, (we have three children of our own) Mom nor Dad has contributed to niece's support since she has been with us, niece is 13 and does not want to go back with mom as mom is pregnant with child #7 and I am afraid if she does go back with her mom it will not be a good environment for her and she may become a "problem child". We have invested more than "money" on this niece, we have invested "emotions" "love" "parental guidance" "time" "family sacrifices" etc. Dad has only seen niece three times in last 9 years. If mom were to pull up in our driveway and said niece had to go back "home" with her but niece doesn't want to go what can we do ?
You need to talk to a local family law attorney in your county. It appears that you have grounds to ask a judge to give you legal custody of this child. I have no idea what "temporary guardianship" means because "guardianship" can only be granted by a judge & that is a probate matter - normally minor children cases are controlled by the Texas Family Code and it's referred to as "custody". If she merely signed a piece of paper, that is not legally binding and she can change her mind at any time.
Please talk to an attorney immediately.
The answer may be that she goes. You should file a Suit to get you rights as a non-parent conservator of the child. To become a legal parent. You can request an order to establish the status quo until the case comes to trial on a temporary hearing so that the child remains with you while it winds through the court process.
You do need an attorney. The parent may based on the limited facts presented in your question have a parental presumption. Only after an in-dept consultation can someone advise you on how best to proceed.
You have not been granted temporary guardianship. People don't grant temporary guardianship. Judges do. If the person with legal custody shows up and demands the child, I see nothing in your facts that would protect you if you refused to return her. You need to hire an experienced family law attorney to advise you on the chances for obtaining legal authority over this child.