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 tell family you have chosen not to have children, use this factual base: explain what is changing, when it takes effect, what remains private, and any practical impact on them.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. Share the news directly and invite only the questions or support you are ready to receive. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respect the boundary already visible
Do not make one recipient responsible for announcing it to everyone or defending your decision. Plan privacy, housing, medical, employment, and personal safety before disclosing a change that may trigger retaliation. A block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.