Your B inlaw has no legal basis in a court because you send him an email. Just make sure that the email is professional and courteous. From the Org's perspective, he is an elder and you are supposoed to contact an elder with any Org related business. Is he just thick? You wouldn't want to appear as deliberately trying to annoy him.
How do you know that you have congregation file other than a publisher record card? What would you want to accomplish by legal action? I don't see it.