Her terminal mentions her travel east from the Abbey to the Commonwealth. From there, she took the Duchess Gambit south to Rivet City where she met up with Father Clifford before finally arriving at Point Lookout.
After you find the Krivbeknih, she will be found dead inside the tent (she was killed by a trio of smugglers for unknown reasons, although it is implied that Obadiah Blackhall hired them as mercenaries to kill her, as she was chasing after his book).
If the player character talks to Marcella about her religion, she will mention the Bible. The Lone Wanderer can then remark that Revelation 21:6 was their mother's favorite passage and even choose to say how cautious they are of cults.
She can recite verses from the New Testament when asked. The verses she recites are John 1:4-5, John 5:24, John 16:33, and 1 Corinthians 15:52.