Power of Attorney. If you mean the power of attorney, poa, you will need your grandparents permission. If they are incapacitated mentally such as dementia, they will need a to perform a competency exam and have an attorney submit it to the court for approval. Some states want 2 doctors to agree and some only one.
Proxy. Do you mean health cae "proxy" ? Generally the patient is the one to designate the proxy- or the person that will make health decisions for a person if they are unable to make it themselves.If they are incapable of designating a proxy, then if you are next of kin you can then become the health care proxy of record by law, .