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You should have a candid conversation with your mother and her attorney about your concerns and see if your mother will remove you as her attorney-in-fact. If you act improperly in your capacity as an attorney-in-fact, that may cause liability to you. However, I cannot see how you can be held responsible for your brother's actions that may be taken without your knowledge/participation. Be sure to consult your own attorney to protect your legal rights.
I agree with Attorney Mashal. You have some valid concerns. On the other hand, it sounds like your mother named YOU, specifically because of the issues (or potential issues) with your brother. After consulting with a lawyer, you may find that you CAN agree to act, if your mother needs you to do so. If that is not the case, you are not required to act. You can either resign, if you have already become agent, or you can decline to act, if that occurs, some time in the future.
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You state that you are concerned about being legally responsible for your mother's expenditures. Unless you agree to accept such responsibility, you won't be just because you are her attorney-in-fact. You would be there to take care of her money and finances, not to use your money to pay for her needs. In fact, depending upon what the power of attorney states, you may even be entitled to be paid a reasonable amount for your services. You would certainly be entitled to be reimbursed for out of pocket expenses incurred on behalf of your mother. The distances may be a problem unless you take care of her finances electronically.
You can choose to accept or decline the responsibilities of power of attorney or executor, but before doing so you should consider the consequences. If you decline and the power then shifts to your untrustworthy brother then he may make bad and important decisions that affect your mother and her estate. If he declines then what do the documents provide as an alternative? If the only reason you do not want to assist with the finances is because you are 2000 miles away, there are solutions to that such as auto bill pay and hiring local people to handle specific financial issues which are probably permitted under the power of attorney document and paid for out of your mother's assets. If you are sure you do not want the future responsibilities then yes, tell your mother so that she can remove you and change her estate plan accordingly.