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 an in-law boundary with your partner, use this factual base: name the current pattern, concrete impact, and decision that needs both partners' attention.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. Ask for a time and format to discuss one issue, then identify any outside support that could help. 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 abandonment, use intimacy as leverage, or demand an answer during an unsafe or heated moment. Use specialist domestic abuse or emergency support when coercion, violence, stalking, or immediate danger is present. A block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.