The holder was given a tough time of things in its 1-0 win against Caen in the previous round but made light work of Sunday's (AEDT) 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, which made it three wins in a row heading into Thursday's (AEDT) UEFA Champions League Round of 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 an opponent 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, which 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.
Elsewhere, 17-year-old Rayan Cherki scored twice for Lyon as it overcame a spirited effort from Ligue 2 side Sochaux, the winner of the competition in 2007.
Algerian defender Djamel Benlamri headed Lyon ahead at home before Maxwel Cornet volleyed in a second to put the hosts in control.
Chris Bedia pulled a goal back for Sochaux only for Cherki to restore Lyon's two-goal cushion before half-time with a crisp low finish.
Bedia's second of the game again gave Sochaux hope but Jason Denayer nodded in from a corner and Cherki completed the victory three minutes from time.
There was one big upset in the round.
Red Star dumped Lens, fifth in Ligue 1, out of the competition at the historic Bauer stadium in Saint Ouen in the northern Paris suburbs, winning 3-2.
Pape Meissa Ba gave Red Star a 21st-minute lead. Winger Benjamin Gomel, who came through the Lens youth system, crossed from the right and Ba controlled the ball and fired home.
Lens took a while to adjust to the artificial surface but levelled when Facundo Medina headed in Ismael Boura's corner at the near post in the 29th minute.
Cheick Doucoure gave the visitors the lead with a powerful shot into the roof of the net in the 49th minute.
Diego Michel, another sub, pounced on a rebound in the 83rd minute after Lens goalkeeper Wuilker Farinez had parried a free-kick.
In the 90th minute, Dzabana met a cross at the near post with a left-foot shot that caught Farinez flat-footed and won the game.
Founded in 1897 by Jules Rimet, the man who created the World Cup, Red Star won the French Cup for the first time 100 years ago in 1921 with its most recent Cup triumph coming in 1942.
In another upset, fourth-tier Le Puy beat Ligue 1 struggler Lorient 1-0.
Lenny Joseph outpaced the visiting defence to finish off a 61st minute counter-attack and give Le Puy, which plays in National 2, victory over Lorient, which is 17th in Ligue 1.
Metz won 4-0 at second-division Valenciennes, while Montpellier shook off fifth-tier Ales 2-1.