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 ask family not to post photos of your child, use this factual base: identify the visit, topic, money, photo, event, care task, or comment that needs a limit.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. State what is allowed, what is not, and how you will respond consistently. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not threaten access to children or elders as punishment, and do not debate the entire family history. Use safeguarding, legal, domestic abuse, or emergency support where safety, custody, or coercive control is involved. You do not need the other person to agree with a boundary for you to follow it consistently.