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.
Sans is shown to be very laid-back, sleeping on the job as often as he takes breaks. He enjoys making poor skeleton puns, to the annoyance of his sibling. He is kind and reassuring, but becomes eerily serious at abrupt moments (particularly when angry), demonstrating that his casual nature is perhaps a guise.