It is eternal friendship
Sabo has never been a real member of the Luffy family, but he, Luffy and Ace shared a drink that made them officially brothers. They seem intimate though not as deep as Ace and Luffy, this is probably because they do not spend a lot of time with each other because he is gone and they never meet again. Sabo still tends to protect or save Luffy when they are hunting together. Although when first meeting Luffy, Sabo wants to kill Luffy for discovering their plans.
After Sabo and Ace fought together against Porchemy to save Luffy's life and they became friends. Then they have time together to steal and hunt. Later, it was revealed that Sabo was the son of a Goa Noble. They have shown a relationship with faith and love since he gave up and returned to his father for Luffy and Ace to be safe from the hired pirate Bluejam. When Sabo's death was reported by Dogura, Luffy cried all night until he fell asleep. Luffy just felt sad when Ace told him to live proud, which is what Sabo wants. In this way, Sabo has contributed to Luffy a resilient and altruistic will.
