Child custody! Urgent help!!!

So, since November, we have had our stepdaughter living with us, bc her mom (we will just call her K) couldn't take care of her. She has come to visit randomly a few times. Well we decide to let her keep my step daughter (let's call her L) for spring break. Everything was fine. K set up a time and place to meet the 10am yesterday. I start driving that way. 1/3rd of the way there she texts me: "Taylor im sorry but dont worry about coming because before i give her back aaron and i have to sign a legally binding contract regarding L. I dont want you to have to make a pointless trip." I say she needs to go to school tomorrow and Aaron will discuss it later. she says "No. She is not going back until its signed and notarized. Im sorry. He will either be in austin to sign it tomorrow or i will have her enrolled ther on tuesday. Im sorry but this is it."

My fiance is 6 hours away working. He can't leave. She can't withdraw her from school but she doesn't know that obviously.

I am limited on what I can personally do bc we aren't married yet. plan on getting married at the jp soon as he gets back.

What can I do?

What is the plan of action we should take?

I have a lot of good info already but I need all the info I can get. we are in Texas if that helps.