What does “sibling-coded” mean?

Hi, I’m a really big fan of Persona 5 and often scroll Reddit and Twitter to look at art of my favorite pairing, Joker x Futaba (aka “Shutaba”). However it seems no matter where I go I find people saying that people who ship Shutaba are disgusting because the characters are “sibling-coded”? What does this mean? I’m confused, I don’t know what I did wrong.

I’m still new to the whole shipping business and all I know from friends is just “avoid proshippers like the plague”, so I was hoping you all could explain this to me.