My mother has dementia. My brother convinced her to change her trust in his favor. What can I do?

I suspect there's more going on here than you've shared, but, basically ..

.. if you have DPOA, first: is it for healthcare and/or legal matters and second: had your mother been determined to be mentally incompetent for legal and financial and medical matters? If so, I'd be getting to an elder's specialty attorney and see if the change is legal. If she's still legally sound of mind, and changed it so that you no longer are in control, hand the keys over to your brother and walk away. Don't get caught up in being a slave.

Thank you LadeeC.
Her doctors have indeed diagnosed her dementia prior to my brother changing her trust. The problem from what I understand; if she is able to answer 4 questions asked by her lawyer accurately at the time she changes her trust it's valid even if a second later she is completely demented.

I'd contact an Elder Law office. If she has dementia, you may need to file for guardianship.

You have to decide if losing the assets of the Trust are worth the emotional energy and legal fees you would have to expend to reverse the changes to the Trust. I recently walked away from a substantive inheritance because emotional freedom is more valuable to me than being held hostage to unfounded accusations and verbal abuse. Having peace in one's life is priceless.