Ok here we go:
So obviously the backs of the three sisters don't have eyes on them that turn people into stone otherwise Luffy would have. It's probably a story they told the villagers because they wanted to hide whatever it was on there.
MY THEORY IS: that Boa Hancock and her sisters have a Whitebeard tatoo on their backs just like Ace does. Maybe on one of their voyages away from the island they ran into the whitebeard pirates and lost to them, ended up joining their crew, etc etc.
- The reason they hide this from the village is so they don't see their leaders as subordinates to some other pirate crew thus the lying about the stone curse.
- Luffy's "seen it before" on Ace's back
- Hancock's real reason for not wanting to cooperate with the shichibukai is because she'd rather work under another pirate rather than the government
We all know that Kuma for some reason saved the Straw Hats. And I think there was some other theory that Kuma was working for Whitebeard or for Dragon? I might be wrong. BUT if this were true, then Kuma most definitely sent Luffy to this island on purpose. AND he probably sent the other Strawhats to other random satellites of Whitebeard's crew. Which will mean that the next time the SH crew reunites is during the fight between the Whitebeard Crew and the Shichibukai. BOOYA!!