Your sister is considered a "fiduciary," which means that she has legal duties and she cannot simply do what she wants. She cannot, for instance, legally place her mother's assets in her own name. That would be considered self-dealing, and a breach of her fiduciary duties. Having said that, there is not really anyone currently looking over your sister's shoulder, in a legal sense. If you feel that she is taking advantage of the situation, the only real recourse is to go to the probate court and seek appointment as your mother's guardian/conservator. I would expect your sister to contest this, so you would want to retain a probate attorney to assist you.
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I am really sorry to hear about your situation. Mr. Frederick provides a great answer. If your Mom is capable of communicating, ask her if she has named your sister as agent under a Durable Power of Attorney (POA). You indicate your mom already named your sister as agent. Unless your Mom revoked the POA or executed a new one, this prior POA is probably still in effect. If your Mom has capacity, she can execute a new POA and name a new agent. If your Mom does not have capacity, or does not want to execute a new POA, things get more complicated. This may leave you with a guardianship and/or seeking an injunction and breach of fiduciary duty action as remedies.
Whether your sister can, or should, be acting as agent depends on whether the POA springing or not. A springing POA only gives the agent power when the principal (person who executed the POA) becomes incapacitated. A non-springing POA gives the agent power to act on behalf of the principal immediately upon execution of the POA.
If your sister is legitimately acting as agent under a POA, her powers depend on what powers the POA document grants her. The POA could give her the ability to sell assets. It might not. However, like Mr. Frederick has stated, regardless of what powers are given, your sister is still under a fiduciary duty. Thus, whatever she does should be in the best interests of your Mom.
If you feel a breach of this duty is occurring, you should consult an elder law attorney immediately. If you believe your Mom is being exploited, you can contact Adult Protective Services. A letter from an attorney to your sister might be enough to deter her from doing anything inappropriate. One of the main methods of cutting off an agent under a POA is to intitate a guardianship. Amongst other things, this requires you to prove your Mom is incapacitated. A guardianship can be very expensive, time consuming and stressful, especially once contested. You need an attorney to guide you, as this is a complicated situation and how to proceed is very fact specific.
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