The holders were given a tough time of things in their 1-0 win against Caen in the previous round but made light work of Saturday's clash at Stade Francis-Le Ble.
Sarabia fired in his sixth goal of the campaign shortly before half-time after Mbappe had opened the scoring inside nine minutes with an impressive solo strike that saw him carry the ball half the length of the field.
Mbappe doubled his personal tally for the day with an unorthodox chested finish to ensure comfortable passage through for PSG, who made it three wins in a row heading into Wednesday's Champions League last-16 second leg with Barcelona.
Mbappe also scored a brace in last week's comfortable Ligue 1 win at Dijon and picked up from where he left off with the opener against opponents positioned 13th in the French top flight.
The France international received the ball just inside the opposition half, carried it to the edge of the penalty area and picked out the top-left corner with a thunderous strike that left Sebastien Cibois rooted to the spot.
Mbappe was denied a second by the inside of the post when played in by Marco Verratti but PSG, who survived a penalty scare after Romain Philippoteaux went down under contact from Abdou Diallo and Mitchel Bakker, soon had a second goal.
Julian Draxler took the ball past a couple of opponents and teed up Sarabia to fire away from Cibois, adding to the goal he scored against Brest in PSG's 3-0 league win when the sides last met in January.
Cibois denied Mbappe from a one-on-one early in the second half and that sparked Brest into life, with Bandiougou Fadiga testing Sergio Rico with a couple of attempts in the space of a minute, the second requiring Ander Herrera to block the ball on the line.
But the hosts' hopes of a comeback were ended 17 minutes from time as Verratti's pass ricocheted off the chest of Mbappe and trickled in from a few yards out.