Legal Question in Family Law in Texas

The father's suit would be based on either a material and substantial change in circumstances OR his expectation that your oldest will tell a judge that he wants to live with his dad. I don't know any judges who care too much about what a 12-year-old has to say, even though the law requires the judge to let the 12-year-old express an opinion.

The 12-year-old does not get to choose where he lives--that's up to the judge. The 12-year-old just gets to express an opinion. But the judge has to decide what is in the child's best interest and, based on the facts you've provided, it doesn't sound like the change in custody would be in the child's best interest. Moreover, the Texas Family Code instructs judges to avoid splitting siblings unless splitting the siblings is in the children's best interest. It almost never is.

I would go ahead and come up with an attorney's retainer and put it in the bank. Then wait for dad to take his move. You'll feel better if you have some money in the bank. But don't give in. Fight.