Featuring a slew of closely tied crews and groups, One Piece loosely defines the word family. On one hand, there are cases like Garp, Dragon and Luffy, in which the same blood runs through their veins. On the other, Whitebeard's huge assembly of pirates challenges the notion of family needing to be related by blood. Luffy's crew undeniably belongs to the latter category. But before the Rubber Man formed his own pirate crew, he became brothers with two other Pirate King aspirants.
Although Luffy comes from a family of strong warriors, the Monkey family doesn't excel at taking care of its offspring. After Luffy was born, Monkey D. Dragon left him to Monkey D. Garp. The latter wished for Luffy to become a formidable Marine officer, so he exposed him to numerous dangerous activities like throwing the child to a deep ravine or leaving him in the wild. After Luffy met Shanks, Garp took him to the mountain bandit leader Curly Dadan, whom he also tasked to take care of Ace. That's where Luffy first met Sabo and Ace; here's how they became the ASL brothers.
When Luffy first met Ace, the latter couldn't be bothered with him. Ace would take extra measures to ensure the younger boy couldn't follow him but due to Luffy's perseverance, he eventually made it to the Gray Terminal -- where he also met Sabo. The two older boys tied Luffy up, resulting in the latter getting captured by Porchemy as they hid from the pirate. Luffy refused to tell Porchemy where the stolen money was, even when he was almost beaten to death. Touched by the boy's naivety, Sabo and Ace went to the Bluejam Pirates' base and launched a surprise attack, saving Luffy and also accepting him as one of their own.
The ASL brothers continued their adventures until someone recognized Sabo. Luffy and Ace egged him to reveal the man's identity, so Sabo admitted the truth of his real status as a noble. The other two accepted Sabo for who he was, and the three kids exchanged cups of sake to commemorate being sworn brothers. Even though the three were able to continue causing trouble, it was short-lived; Sabo promised his father to leave as long as he wouldn't let the others get hurt.
The brothers faced further devastation in One Piece after learning of Sabo's supposed death at the hand of a Celestial Dragon, and were completely separated when Ace sailed away after turning 17. The next time they met was when Ace greeted the Straw Hat Pirates as they fled from Smoker. But another tragedy befell Luffy when Ace died during the Paramount War, leading Luffy to believe both his brothers were dead.
During One Piece's "Dressrosa" arc, Luffy was pleasantly surprised to find that Sabo was still alive and kicking. The latter lost his memories and had only recently regained them, which was why he wasn't able to aid Luffy during his attempt to rescue Ace. Sabo asked for Luffy's permission to eat the late Ace's Devil Fruit, which the latter delightfully approved, and assumed the Lucy persona.
Sabo participated in and won the tournament, ultimately inheriting the powers of the Flame-Flame Fruit. Even though Ace is no longer with his brothers, his will and Devil Fruit powers live on within Sabo, who also pledges to protect Luffy from then on.
From its introduction up to the present, the ASL brothers continuously faced hardships and a constant threat of death -- especially since Sabo is once again rumored to be dead and Luffy is facing two of the Four Emperors. They may be down to two, but their brotherly affection toward one another remains the same. Similarly, many One Piece fans are rooting for their success and happiness.
