Was Sabo Ever Really Meant to Be Ace and Luffy's, Brother?
The term "family" does not just refer to biological relatives in the vast and complicated universe of One Piece. In reality, the majority of the families in the series are formed through maritime exploits, with the Whitebeard Pirates being possibly the most prominent example. Of course, the Straw Hat Pirates had a similar type of connection, but Luffy had previously established a pact with a few other characters, with whom he finally became sworn brothers, before meeting his current nakama.
Ace's entrance to the franchise enthralled fans, with many of them taking an instant affinity to the character. He was shown as a strong and reliable person who was constantly concerned for his younger brother. The fan following was naturally saddened when the Fireman was killed. However, another of Luffy's brothers was introduced after the time jump, creating questions over whether his inclusion was always planned or was simply a whim to capitalize on Ace's fame.
The existence of Sabo was hinted at much earlier in the Paramount War Arc.
The discovery of Sabo's existence surprised people, especially after Ace's death, because it was previously considered that Luffy's only brother was the Whitebeard Pirates' second-division captain, Portgas D. Ace. Nonetheless, Sabo's creation was not the result of a whim on the side of One Piece's designer. His disclosure was foreshadowed in previous arcs before to his debut appearance in Dressrossa, notably during the "Paramount War" arc.
Ace didn't want his brother to know about his abduction by the Marines since he was well aware of his brother's inclinations. When Luffy arrived at Marineford to save him, it was inevitable that the two would talk about Luffy's deeds, which occurred in Chapter 558 of the manga. Ace advised him to butt out of his business and flee, but Luffy refused. The rubber guy insisted on reminding Ace that he was his brother. When he stated that, there was a brief flashback of them drinking sake together. However, what many people may have overlooked at the time was that there were more than two glasses there. There were three, which was a definite indication of Sabo's presence.
In terms of his relationship with Luffy, Sabo takes over Ace's role.
While this demonstrates that Sabo's appearance was planned, it does not affect the fact that the Revolutionary Army's second-in-command assumes the role of the late Ace. He exhibits a similar amount of protectiveness, caring, and support for Luffy that Ace previously did. Sabo's encounters with Luffy also demonstrate his everlasting commitment and readiness to go to considerable measures to safeguard the well-being and dreams of his younger brother.
There's also the Flame-Flame Fruit to consider. When Ace died, his potent Devil Fruit resurfaced. Sabo eventually won the privilege to devour it, obtaining the Devil Fruit abilities of his late brother. Sabo's newfound talents allow him to carry on Ace's fiery heritage while also adding his own touch to the mix. It's a symbolic gesture indicating that the torch has been handed from one brother to another, thereby preserving the spirit of adventure and camaraderie in One Piece.