Children feel the need to become caretakers and protectors, but the parent objects, resists and, in the most difficult cases, rejects not just the child's interference, but the child as well. Resolving such situations is difficult because our control is minimal and situations are so hard to interpret. Start with objectivity. With all the honesty possible, we can try to determine which parental behavior is merely a nuisance and which is a real danger to the parent or to others. This is not a time for vague stories or opinion.
Objectivity requires facts, cold facts, a diary of incidents and dates.
If another sibling or family member shares your concerns and responsibility, ask them to do the same. If Mom or Dad's behavior is only the natural diminishing of ability that comes with age, and is not clouded by impaired thinking or real physical limitations that they can't or won't recognize, the prudent and loving thing to do is figure out how to live with the behavior. Appreciate the opportunity to help them as they once helped us. Clean up after their mistakes. Complete their unfinished jobs.