A defamation guru, huh?
Well, I am not sure any advice I can offer here will be all that marvelous. I am a bit puzzled by few things, things which may or may not make a difference to what recourse, if any, is available.
For one thing, kato, I am not sure what you mean by a "government envelope." It is unclear from the information you provided whether others beside yourself received similar letters. You also do not mention
how the information in the letter was presented (as a legal notice, a threat?).
If this letter was sent to you by a single individual and you were the only one who received a letter with false accusations, and you know that you, your wife and your siblings committed no crimes, then there has been no actionable defamation. No one's reputation has been injured.
You do not mention if any threats were made in the letter (threats of physical injury or threats to expose the false accusations to others or threats of legal action). If there is only the
one letter sent to
only you by seemingly only
one person, and there are no threats contained in the letter, then it would not be harassment, which generally requires
repeated acts.
Based strictly on what you have posted here, I do not see that you have any legal action you can consider - especially since there is no clear proof of who sent the letter anyway, only your suspicions as to who sent it based on a postmark.
If you want to show the letter to your local police, you could always do that, not that they will, or can, do much about it - unless there is more to your story than what you have indicated here.