You say they have sibling-like relationship, but Oda has always made it known who has a "sibling" like relationship. Luffy doesn't refer to anyone on his crew as a brother or sister but happily brings up his brothers Ace and Sabo when he has a chance. If Luffy or Nami had ever referred to each as a brother or sister then I'd understand but they haven't. Meanwhile, Nami has referred to Robin and Franky as "onee-chan" or "aniki", respectively, before but has never done this with Luffy. Another example, though not a ship, would be Chopper and Carrot. She refers to him as "choniki", Chopper and aniki...so if Luffy and Nami had a sibling like relationship, Oda would have made it known. Also, Genzo, Nami's foster father (Oda said he was her foster father in a SBS), making Luffy promise to not take her smile or else he'll kill him is reminiscent of a father giving her daughter's hand away in marriage...personally this is something I've had to stress with my own little sisters when it...