Sanji hadn't met Usopps father in any official way. The man hadn't bothered to be there when Sanji married his son and while he'd been pissed at the time, Usopps later attempts to calm him down by saying he hadn't even seen his father since before his mother died only infuriated the blond more.
Usopp was proud of his papa. Sanji just wanted to kick his teeth in.
But here it was, a chance meeting. The marimo pack mule and himself gathering supplies while everyone else waited back at the ship to shove off and along came the Red Hair Shanks crew from the north side of port.
Sanji sent Zoro back to the ship with what they had so far before he spotted them. He casually followed them into the tavern they chose and sat down right next to the man so obviously his lovers father even without the telltale nose.
After a drink or two and Yasopp was talking about how the call of the sea and adventure seduced him away from his home.
"You know…I know about the call and seduction of the Blues too. One in particular." Sanji started as he snuffed out another butt in the ashtray. "But I would never leave my partner alone to suffer without me. I wouldn't leave my child that idolizes me. If I loved them as my family, I would protect them, no matter what siren song fucking water was whispering in my ear."
"Well that's you buddy."
Sanji didn't really mean to kick his father in law to the ground. The first mate held him back anyway.
"It's okay we're family."
"Family?" Yasopp laughed in denial.
"Here's a hint, Your son is on a ship about to shove off, I really should be getting back to him."