My siblings don't want our 84 year old mother tested for dementia.

My mother was diagnosed in 2023 with a mild-moderate cognitive impairment by an OT, with her primary care doctor requesting neuroimaging. My siblings will not agree to the imaging, and my mother is siding with them, making me out to be the "bad guy" in the family. It has been quite obvious to me since about 2018 that she is in cognitive decline, however, my siblings do not take the incidents seriously enough, and are able to deflect individual anecdotes that "sometimes we all do that." I am in need of advice!! I am concerned for her future care, and what could be a rapid depletion of resources if we are not properly planned.