Kata and shiryu aren't 2nd in command though. That's Perospero who is the actual 2nd of the big mom pirates, and Lafitte, who has the same job as Sabo.
This is a list of the 2nd strongest fighter next to the captain that's all
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Marco also isn't second in command, the whitebeard pirates were explicitly not ranked by power or authority, just numbers. They're an equal family.
The Big Mom pirates said it's Katakuri and not Perospero who should become new captain (was it Daifuku?). He is the most respected member internally and externally. Big Mom though has anger, psychologial and hunger issues as well as no regard for her family/crew. They don't respect her, they fear her. And she fears someone capable - Katakuri - coming for her spot. It's like a manager at work putting down the most talented staff member in order to keep what little power the manager has.
BM - Streussen (but he is to old nowadays) so Katakuri
Roger - Rayleigh
Whitebeard - Oden (but he defected and died) so Marco
Kidd - Killer
Crocodile - Nico Robin (but she defected) so Daz bones
Dragon - Kuma? Aokiji? Hidden member? I don't think Sabo is it, simply to young. Ivankov doesn't seem respected enough by Dragon. But as there is no one revealed yet it is Sabo
Luffy - Zoro
Law - Bepo (should seriously change. Then again I'm not sure Law is a pirate, have a feeling he is Sword)
Doflamingo - Vergo
Kaido - King
Shanks - Beckman
Wiper - (the guy Enel defeated, mohawk dude)
Fisher Tiger - Jinbei
Capone - Vito
Arlong - Kuroobi
Bellamy - Sarquiss
Yet some here would claim Miss Doublefinger could have been VC or that Diamante, Pica, Trebol and Vergo hold the same value when it's clear Vergo is above. It's clear Kuroobi was above Hacchan and Chew.