Make the first sentence usable
Lead with the reason for writing so the recipient does not have to decode a long emotional introduction. For a message about how to set a boundary about lending family money, use this factual base: identify the visit, topic, money, photo, event, care task, or comment that needs a limit.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. 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.
Respect the boundary already visible
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. A block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.