If you look at the ATF'S q/a section it covers this. When hunters travel to huntingblocations, they many times mail their rifles ahead a few weeks to keep from dealing with flying with them.
This is legal if the rifle is locked into a case with the key only possessed by them,(without key access by reciever, there is no transfer of possession).
So, a firearm found in the mail with the sender being your mom sending the reciever being your mom shows no transfer and therefore legal.
When I do this, I send to myself C/O my buddy,.when I show up, I have the key to the case which makes it legal. No transfer has taken place.
So if this is your mama's carry gun, how was it forgotten in the safe. Most carry guns never see the safe as they are.....carried.