They should have a firearms unit, if not then ask for an appointment to speak with the sheriff.
Tell them that you are planing to buy a firearm and that you live with your sister who has a felony on her record. Explain to them that you want to be lawful for your sisters sake as well as your own.
Tell them along with the purchase of a firearm you will buy a gun safe that will be stored in your bedroom that has a keyed doorknob lock as well as a deadbolt lock on the door.
If they can't give you any legal advise then you will have to consult with a local attorney that is familiar with gun laws in your state.

