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 apologize for excluding a relative, use this factual base: describe what you did, the family context it affected, and any responsibility that still needs attention.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Make a direct repair to the affected person and change the practical pattern that caused the problem. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not invoke family loyalty, childhood history, or a holiday deadline to force reconciliation. Keep children and vulnerable relatives out of adult conflict and follow any safeguarding or court requirements. You do not need the other person to agree with a boundary for you to follow it consistently.