User blog comment:LuNa/SBS volume 67/@comment-4493594-20120911080634

"Oda gives us some backstory for Sharly! Arlong was abandoned by his father is Fishman District when he was very young. He doesn't remember his father's face. Then when Arlong was 15 a man claiming to be Arlong's father abandoned a little girl there as well who was born to a different mother. When she was 4 she foretold the coming of the proceeding year's Great Pirate Age."

Nice. That might explain no interaction between them whatsoever.