Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to discuss power of attorney with family, use this factual base: record the care task, health instruction, schedule, cost, decision authority, and current risk.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Assign one decision or task at a time and confirm who will do it, by when, and with what information. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not remove the cared-for person's voice, shame a relative, or promise care you cannot sustain. Use clinicians, safeguarding, emergency services, or legal advice for neglect, abuse, incapacity, or immediate danger. You do not need the other person to agree with a boundary for you to follow it consistently.