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 support someone grieving a sibling, use this factual base: use the person's name and relationship when known, and avoid details the family has not shared publicly.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. Offer one concrete form of help and remove any expectation of an immediate response. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respect the boundary already visible
Do not say everything happens for a reason, demand strength, or center your own similar loss. For immediate risk, traumatic circumstances, or a person unable to stay safe, use local crisis or emergency support. A block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.