Stop being mean to my baby

Of the ten main cast members, six of them come from adopted families, in addition to many more characters in the supporting cast

What is widely regarded as one of the best arcs in the entire series (marineford) revolves around a character trying desperately to save his adopted brother from execution. The entire world knows that they're not blood related, but even the marines (their sworn enemies, the people trying to kill them) still refer to them as brothers

One of the four strongest pirate captains in the world refers to his crewmembers as his children, and they all call him their father

One of the main characters looks his biological father in the eye, and tells him that the only reason he's still alive is that he didn't want his (adopted) father to be disappointed in him for letting his bio family die