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.
Papyrus takes on the older brother roles of cleaning, cooking, and work. Sans is super lazy and adopts the role of little brother. I'm not sure just where I found this, but I've heard before that the two are fraternal twins, or brothers given birth to (or created) at the same time.