A cleaner noticed them on Monday and called police after ringing the doorbell and receiving no reply.
The police entered the building and called out if anyone was there and the nuns replied: "Yes, we are here, in the elevator."
The two women later said they had "prayed so much" while trapped.
They were taken to a nearby hospital where they were treated for dehydration.
The Marist Sister House host visitors to Rome from religious missions around the world.