Sissy (or Cissy, as the girl on "Family Affair" spelled it -- I think she was a Catherine) is an OK nickname for a girl. Sissy Spacek (christened Mary Elizabeth) doesn't seem to have a problem with it -- and she's 60. It's sort of the female equivalent of Bubba (brother). But for a guy -- no.
As for the OP's nephew, Prissy Boy is one of those nicknames that has outgrown its use, IMO. It might have been cute when he was a toddler, but it's time to retire it, or change it to PB. (If his friends overhear family calling him PB, and they ask, he can always tell them it's short for Peanut Butter.)
Bagman has a nephew named Matthew. His childhood nickname was Fergie. Matthew evolved into Massey, someone made the connection to Massey-Ferguson (tractor makers), and Fergie evolved from that. I'm not sure exactly when the family stopped calling him Fergie, but he's been Matt as long as I've known him.
He also has an old friend nicknamed Chub. He was a junior and inherited the nickname from his dad, who *was* chubby. Chub the Younger never has been. He was on the gymnastics team in high school and may be the only gymnast in the history of the sport called Chub! (Then again, only those who knew him back in the day -- like Bagman -- still call him that.)