Can the parents family doctor declare the parent incompetent?

It takes a judge to declare anything for legal purposes, but a doctor is usually ethically and legally obligated to report any and all signs of abuse, neglect or mistreatment to local authorities. In the case of minor children, that would usually be social or family services. Family or social services would then intervene and pending an investigation may ask a court/judge to act in the best interest of the child. That could include declaring a parent incompetent and remanding the minor child into the custody of either a foster home or relatives until the parent/s can show that they are again able to adequately care for the child.