If your mother lacks capacity she cannot perform any of these actions legally. You need to hire an attorney to protect her interests and very likely that would involve a guardianship proceeding to protect her assets and perhaps recover anything that was done improperly. Run, do not walk, to a lawyer before any of her assets are squandered and become harder to recover.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/
First, joint tenant and TAO are two different concepts, both of which would unlikely be put on your mother's retirement account, so first I'd recommend you consult with an atorney so that concepts can be understood. For example if another name is put on a retirement account, that would likely be a taxable withdrwal and have adverse tax afects; another example is TOD is not normally available for retirement account (a beneficiary desination, siminlar to TOD but not a TOD).
Your inquiry should ask about court supervision, lioke a guardianship, and een tough your brother's appointment might be presumed, if there are facts that show the matters you set forth, you might prevail with an independent guardian and gettin the POA revoked by the court. Medical testimpny would be required to show not competent or otehr attack valid when the retirement account is changed. My recommendation is to meet with an attorney as soon as possible and consider taking immediate court action depending on the facts and ircumstances (or approaching first and determine whether can be resolved without cort action - dpends on your brother).
That's my thoughts; hope they provide some assistance to you.
You need to retain counsel to assist you in possibly filing a guardianship action to protect your mother and her property from your brother.
Legal disclaimer: This answer does not constitute legal advice. I am admitted to practice law in the State of Missouri only, and make no attempt to opine on matters of law that are not relevant to Missouri. This answer is based on general principles of law that may or may not relate to your specific situation, and is for promotional purposes only. You should never rely on this answer alone and nothing in these communications creates an attorney-client relationship. less