No, you cannot be sued for becoming your grandfather's agent under a POA. But you cannot complete the POA. It is up to your grandfather to do so. He can pick anyone he wants. If he has capacity, he should definitely have a durable POA for health and financial affairs. (The "durable" part is important, because it means the form will still be in force, even if your grandfather becomes incapacitated.)
If your grandfather does NOT have capacity to sign a POA, then you would need to seek probate appointment as his guardian/conservator.
I would not worry about his sister, but I WOULD retain an attorney to prepare the estate planning documents and/or represent you in the court proceedings.
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It is difficult to say whether or not someone would take you to court or not, as many frivolous legal actions are filed. However, it is certainly true that it is perfectly lawful for your grandfather, if he is competent, to make you his attorney-in-fact by executing a POA. It is advisable to have the documents prepared by a competent attorney who can prepare the documents in accordance with your grandfather's needs and advise your grandfather on durable general POAs, as well as other important documents, such as wills, living wills, health care POAS etc. Please mark this helpful if you found it to be so. Good luck to you.
The foregoing is a general answer based upon limited information, should not be construed as legal advice , and does not create an attorney-client relationship. It is the opinion of the writer alone. The author is licensed in Indiana and Ohio attorney only.
It sounds like the doctor was strongly suggesting that your Grandfather get a durable power of attorney or advanced directive in place before he suffers another stroke. That is a good idea, but as Attorney Frederick has said, it is up to your grandfather to decide to give someone the authority to act for him under a power of attorney. If he doesn't want to give someone that power, it is his choice but next time he may not be so lucky and a guardianship may be needed. I have found that many seniors confuse the two. In a guardianship the court takes away much of the senior's right to control their life. With a power of attorney, no one is taking those rights away. Contact an elder law attorney in your area. Most are willing to make nursing home calls.