Siblings cant sign a form to get power of attorney over a parent. Only the parent can grant power of attorney. If they dont have capacity, they cant grant power of attorney. You should consult and elder law attorney.
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Attorney Shultz is correct. There is nothing your siblings can sign to legally give themselves power of attorney for your parents. If the situation is bad enough and you suspect they are abusing your parent, contact Adult Protective Services.
From your description it appears your mother lacks capacity to sign a power of attorney. A person cannot "get" power of attorney over another person. Only a compentent adult can grant power of attorney to someone else. It appears you may need to seek guardianship and conservatorship to accomplish your purpose. You will need to seek assistance from an elder law attorney.
The short answer is your parent must be competent to grant someone power of attorney to act on their behalf according to the terms of the power of attorney. If they were not competent then technically the power of attorney is void, but you may need to prove that they were/are not competent for it to be voided. Probably the best way to "prove" they are not competent and to obtain the best protection for the parent is to pursue a guardianship and/or conservatorship through the probate court in the county in which they reside. However, guardianship and/or conservatorship proceedings can be both difficult and sometimes contentious; therefore, you need to prepare yourself accordingly.