Per 18-USC-922(g)(1):
(g) It shall be unlawful for any person--
(1) who has been convicted in any court of, a crime punishable
by imprisonment for a term exceeding one year;
to ship or transport in interstate or foreign commerce, or possess in or
affecting commerce, any firearm or ammunition; or to receive any firearm
or ammunition which has been shipped or transported in interstate or
foreign commerce.
Note the phrase "or ammunition". This means that if the convicted felon is in possession of just one round of live ammunition that was not manufactured in the state where the offense occurs, that person is guilty of a federal felony. And yes, the US Attorney's office will prosecute for just one round of ammo.
Possession can further be defined as "actual" possession (on his person) or "constructive" possession (within his area of control). Some good examples of constructive possession are the gun in the glove compartment of a car driven by the felon or a "biker chick" carrying a gun for "her man." Having the firearm in the same residence as the felon would certainly meet the requirements for constructive possession if the felon. There is a "knowingly" requirement that might help if it is possible for the firearm to be kept an absolute secret, but that is for you and your family to decide. Having the gun in a safe might satisfy the "ability to control" part of constructive possession, as long as the keys or combination to the safe are never, ever within his "ability to control."
It would be better to not allow the firearm back into the house as long as the felon is there. Perhaps an explanation to your family about "aiding and abetting" and "criminal forfeiture" would help.
Len in PHoenix