Your Mom can revoke the POA in writing and appoint someone else, You may also want to have your Attorney file a Motion to Compel a Fiduciary Accounting along with a Motion to Replace the Trustee for Breach of Fiduciary Duty. See 'Find-A-Lawyer' at the top of this web page. Good Luck to all of your Family.
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You would have to go to Court to remove the Trustee from the SNT. But, given what you have said, the Court should agree with the removal, as self-dealing is a breach of the Trustee's fiduciary obligations. If there is a Successor Trustee designated in the Trust, then that person will succeed him, and if none, the Court would have to appoint someone, perhaps an independent professional fiduciary.
The POA is a different matter. Does your mother have capacity? Only the Principal can revoke the POA. If Mom does not have capacity or the desire to do this, then I am afraid you are going to have to go to court and establish a Conservatorship of the Estate to accomplish this.
You state your mother is of sound mind so she can remove your brother as POA. You will need to petition the court to change the trustee if the trust is funded and active.
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