I really want this to be a thing.
But there might not be a really good way to pull it off with the way site/game rules work right now. Especially when you consider the amount of extra work that would be put onto Staff versus what there is to gain from it.
That being said, creative problem solving can be fun, and I want this, so I'm gonna offer an idea.
There are already certain players who have been around for a while, who are trusted enough to make their characters natives of Withmore. In that same vein, certain players could be trusted enough to apply to a "Family Shell." More on what a "Family Shell" is in a bit, but doing so basically means "I really want my character to be part of an IC family, and am willing to either make concessions on my character's backstory to do so, or keep my character's backstory as neutral as possible to make that happen."
So, a "Family Shell." This is basically... Mother and Father with staff-chosen first names and Family name. Staff decides up to how many children can be in this family. Staff decides where this family was located; likely in a different sector of Red. Staff decides what the mother and father did for work, and the general disposition of the mother and father. Disposition as in... Were they neglectful? Kind? Abusive? Addicted to only 2 kinds of drugs, or 5?
Then players may submit a "Family Shell" application via e-mail, but players will not know any IC details at all. All the player knows is they are applying to be part of a "Family Shell."
And there's a time-window to operate on here. So maybe a specific "Family Shell" is open to application for 2-3 weeks or something. If you don't apply within that time-window, you don't get to be part of the family.
But if you make it in and Staff approves your application, Staff then e-mails you what your character's surname will be, and at this point you are 100% locked in to this. And I guess staff would also e-mail you the names of your siblings....?
Look at that convoluted mess of a suggestion I wrote up there lol. I think that convoluted mess would be the best shot of having actual siblings in the game, and I don't really think siblings would be worth that much trouble.