If there's a HCPOA or Grandma has signed a HIPPA form granting someone access to her medical files, you may be able to get a statement of incompetence from her PCP. With that, you can apply for guardianship or activate a POA if they exist.
I’m assuming that you've made this decision because she has gotten to the point she needs 24/7 caregiving. Was she previously living on her own? Are you planning to have her live with someone or in a facility? Have you put plans in motion as to what will happen when she does move closer? Does anyone have POA? If not and she is deemed incompetent, someone will need to get guardianship.
If Grandma is violently opposed to moving as many Seniors with dementia are, and even those without dementia are loathe to leave their homes, you may need to have her Baker Acted. That you would even consider it makes me think Grandma is out of control. It may be your only option.
Can you provide a little more about the situation, such as is there a DPOA and HCPOA, how far advanced is her condition, is there available care where she is and how are the family members she would move closer to impact her daily care?
If Grandma is violently opposed to moving as many Seniors with dementia are, and even those without dementia are loathe to leave their homes, you may need to have her Baker Acted. That you would even consider it makes me think Grandma is out of control. It may be your only option.