I'm very sorry to hear about your situation!
Under TEC 751.104, as you've read, only the principal can request an accounting, so you are are not able to do so. This is different than the rules for requesting an accounting of a deceased person's estate, in which any interested party can request an accounting. If your mother is not incapacitated, she would have to be the one to contest the declaration of incapacitation. I am not entirely sure why the Judge allowed the clause to stand because that's significantly against your mother's interests.
At this point, your option would be to contest sibling 2's ability to be your mother's guardian of the estate if you can show that S2 colluded with S1 in covering up S1's bad actions as POA.
Now, the bad news is that this is not an area that lawyers will do pro bono work in most of the time, so you are not likely going to find an attorney who will help you with this for free. Also, in Texas there are not court agencies that will help you with this, in fact all court staff are forbidden from giving you any legal advice as that would be practicing law without a license.
You would need to hire an attorney to help you file a counter petition to have your siblings removed. You could attempt to do this on your own, and your local courthouse may have a law library that would have the materials you need, but elder law is a complicated area to practice in so your results trying to do this pro se can't be guaranteed.
If your mother's health is being neglected, you can ask Adult Protective Services to check in on her and make sure that she's being cared for. If they find any reason for concern, they may also get involved in her case legally.