If your brother has healthcare POA (medical proxy) then he is responsible for making healthcare decisions, and for seeing that they are carried out.
What are the reasons that you suspect your mom might have dementia? What behaviors have you noticed that worry you? Have you discussed your concerns calmly and in detail with your brother? What is his reaction?
Are you your mother's primary caregiver? Does she live with you?
Are there other people who have observed your mother's behavior who might be more convincing to your brother?
Give us a few more details and we may have some suggestions.
What are the reasons that you suspect your mom might have dementia? What behaviors have you noticed that worry you? Have you discussed your concerns calmly and in detail with your brother? What is his reaction?
Are you your mother's primary caregiver? Does she live with you?
Are there other people who have observed your mother's behavior who might be more convincing to your brother?
Give us a few more details and we may have some suggestions.