A Guardian manages the care of an incapacitated person's health care and finances with court oversight. Furthermore, a person holding Guardianship trumps one having Power of Attorney. Finally, please know that Powers of Attorney are revocable and, thus, your sister's family members could cause her to revoke yours at any time.
As a result, based on the facts you presented, I believe you should retain an attorney to help you obtain Guardianship over your sister as soon as possible. Good luck to you.
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You received very good advice. In theory, a power of attorney that allows you to handle your sister's financial needs should be enough to protect your sister and her assets. Unfortunately, I see far too many cases where siblings get the incapacitated adult to revoke the existing powers of attorney and sign new powers of attorney. While these new powers of attorney can be challenged in court, it's a fight. And even if you win, any money that may have been taken from your sister will be spent and unrecoverable.
With a guardianship (handles personal issues) and conservatorship (handles financial matters), you will essentially become your sister's parent and be responsible for her physical and her financial well-being. Just like a child cannot sign a contract without her parents' permission, your sister will not be able to sign a contract without your permission. You will be able to stop family members who try to reach her bank accounts.
There are some additional responsibilities that come with being appointed a guardian or conservator. For example, you will have to make periodic reports to the court as to how your sister is doing and where her money is going. Georgia publishes a handbook for guardians and conservators explaining what their duties are. You may want to see whether Pennsylvania has a similar publication. And while you may be able to file for a guardianship without an attorney, you may want to purchase an hour of time from an attorney to understand the guardianship process and to explain the pitfalls of proceeding on your own, especially if you think the guardianship will be challenged.
Good luck to you and your sister!
Unless you came into my office, signed a contract and wrote me a check, I am not your attorney, and you are not my client. This is not intended as legal advice and should only be used for informational purposes only. You should never believe any information that you receive on the internet, especially information that is probably being provided in the late evening hours when I should be sleeping.