Yea, I'm not sure. It's confusing because you have Alluka who's clearly not shown in this picture yet their is a sibling who has left the family apparently and one who joined the Ryodan just to find him. So I'm not sure when that will have a play in the current story. We might find that out at the end of this arc.
There is definitely 5 boys and one girl. Alluka is the one who was revealed just recently.
Actually now that you think, it does looks older. Maybe He's the one Kalluto is talking about.
Still wouldn't make sense with the whole continuation of letters of the Zoldycks. Illumi-Milluki-Killua-Alluka-Kalluto
And that even sounds good xD Maybe Killuki is a rogue between Zoldycks. He committed a sin so grave that no one is allowed to talk about him.
There's absolutely no indication a sixth children exists, but if Feitan is really one of the Z, he'd be considered a disgrace to the family since he's shown as a character who lets anger get the best of him. His special ability can kill friends and foes alike and he doesn't hestitate to use it. That's probably the biggest sin you can commit as a Z. I know the Zs are willing to sacrifice themselves to get the mission done, but you're talking more about getting killed by friendly fire here.