And then he can begin inquiring into their true names. It seems just crazy enough to be true and if it is true, does one lead them or are they more of a cohesive unit? Let's say they do have a leader and there is one Joker that reigns supreme.
Many comics have said that Sans is the older brother... But I haven't gotten any proof yet. It makes sense that Sans could be the older brother... But it makes just as much sense that Papyrus could be the older brother.
Actually, Toby Fox tweeted that in a Japanese version, Papyrus would call Sans aniki, which is a reverent way to address an older brother. And Papyrus' username hints he may have been born in 95, so he's likely between 15 and 24 during the story, which takes place in 201X.