Well, I won't berate you about having betrayed your family as you already know that.
I think returning the money anonymously as E.T. suggests is a good idea if you think your bonds with your wife and your father in law would break by confessing (what I would suggest).
Especially if you were asked before and denied any knowledge of the money's whereabouts.
Should you decide to go with E.T.'s suggestion keep in mind that you also keep on the look out for security cameras that could pick you up while dropping in the envelope.
You also shouldn't print out the note with your own printer either!
They print a unique ID code not visible to the human eye on every page nowadays.
Maybe an internet cafe,an old school typewriter or some kids text stamping set?(that you get rid of afterwards)
The more you stole, the more diligent you should be in returning the money.
I say this because a) you might get discovered, ultimately caught leaving the money or b) the money,note or envelope might be traced back to you if your father in law is very adamant to find out who stole from him.
You also need to have a plausible reason ready why you are / were at the mailbox of your father in law (or wherever you leave the envelope) in case someone spots you, maybe even recognizes you.
Also, do other employees know where their boss lives?
If not it is even more important to leave the envelope where it isn't easy to deduct that you were the one leaving it there.
At least I assume (hope) you learned your lesson and in case of future financial problems you include your family and ask for their help if your relationship is such that this is possible.