My goodness. You are certainly in a very tough spot!! My hat is off to you, as you have certainly been a wonderful grandmother and do not deserve what you are going through.
I do not know what the laws are in TX, but in other states, I know that adoptive parents get monthly "child support," to help them care for children who would otherwise be the responsibility of the state. You have probably already explored this, but if not, your caseworker should have informed you of it. Also, you should be able to get energy assistance and child care assistance and a free phone. And, with a disabled child, you should be able to get payments as well, from the state and respite care when you need a break. Please let me know what you have tried and I will try to help you find resources. I wish I had the funds to help you.
I will certainly be lifting you and these angels up in prayer!