12 years old... then the Court "shall" hear the child's wishes. That does not mean the Judge has to abide by the child's wishes.
In custody battles it really helps to have an attorney on your side to help rebut the allegations that come flying, like here. I would sit down with an attorney and discuss the situation - make sure you have a fair shot. The fact these two children have a tight bond works in your favor, but certainly is not determinative.
Disclaimer: This answer is intended for informational and educational purposes only and should not be considered legal advice nor forming the attorney client relationship.
I would caution you not to put all your eggs in the basket of what you THINK you daughter might say. That strategy could backfire.
Kids will surprise you, and oftentimes a judge, with what they say or that they might like to live with the other parent for a while.
Please do yourself a favor and consult an attorney like Mr. Harding recommended.
I agree with Mr. Harding.
Unless there are some wicked facts you haven't told us here, her father is going to have to prove by clear and convincing evidence that a material change in the child's circumstances demands a change in the custodial relationship.
You need to file a motion for the judge to confer with the child in chambers. In your motion, ask that NO lawyers be present and that NO court reporter be present. Simply let the court reporter note the fact of the conference and let that be it.
Mr. Harding is right about needing an attorney. These are complicated cases made all the worse by meddling ex's who just throw everything, including the kitchen sink, into their crazy cases just to see if anything sticks.
I am very familiar with the courts in Dallas and Collin Counties and know that they take these cases very seriously and will give him a fair chance to present his case, so you will need to be armed to fend it off.
If you don't think you can afford an attorney, call Legal Aid of Northwest Texas and the Dallas Bar Association Volunteer Attorney Program. They might be able to get a free attorney for you. If that doesn't work out, contact the Texas Legal Services Center (www.tlsc.org) and they will refer you to a private attorney who will take your case at a significant discount if you meet their income guidelines.