1) There's no foreshadowing in Luffy placing his hat on Nami. Following this logic Luffy should be related to Shanks and Shanks to Gol D Roger, which is kinda pushing it. So no argument.
2) Luffy placing his hat on Nami is a sign that he deeply trusts her...just like he trusts everyone else in his crew. Following that logic everyone else of the Strawhats is related to Luffy. Again no argument here.
3) Her punches don't actually hurt, it's for comic relief. I'm too lazy to check it out, but I'm pretty sure there are more characters who have hit Luffy like that. And in regards to Garp, he knows Haki. There's an easy way to find out when it's not meant to be for comic relief pre-time skip and that's when they specifically bring it up that Luffy is rubber and that it shouldn't hurt, which was the case when Garp hit Luffy after Enies Lobbies. During the Marine Ford arc Garp hit Luffy and Marco in phoenix mode and those were definitely not just punches of love. Again no argument here.
4) Enel, Big Mom, the Mink Tribe, Thunder Lord McGuy...all of them are characters who have a fighting style that share some similarities with Nami's. So once more no argument here. Not to mention we don't know what kind of powers Dragon might or might not have as that's still pure speculation, let alone "be identical".
So yeah this is completely baseless. There's also no point whatsoever in the two of them being blood-related, it doesn't make any sense from the perspective of the story.
