I just realise something I didn't think of. Its that most of luffy's crew even those with heritage are mostly outcast in their own way when they were recruited by Luffy. Sanji, disregarded by his family. Nami, part of arlong crew at first. Chopper, a monster by both his kind and humans. Ussop, the liar of his village. Robin, pretty straightforward. Franky, the rebel shipwright.
Long story short with exception of the late arrivals Brook & Jinbei the exact origins we don't know about yet, all the Straw-hats share a somewhat tragic but also some-what wholesome upbringing. They all lost parents or lacked a traditional nuclear family in one way or another.
The wholesome part is that every single one of them eventually found a surrogate 'family' that meant a great deal to them. They may not have been families in the traditional sense but they all formed very strong connections.
Now they're all part of the Straw-hat family. The difference now is that they're all capable of making their own choices, not children anymore and they've all chosen to stay with the Straw hats. (Or return to them, Usopp, Sanji, Jinbe? w/e.)