Mr. Curtain when he disguised himself as Mr. Benedict in book two:

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Mr. Curtain when he disguised himself as Mr. Benedict in book two:
A requested sketch of our associates, The Mysterious Benedict Society. Last seen: in a train station in Lisbon. . . . . . . . . . . . . . . . . #vsco #vscosketch #vscoart #themysteriousbenedictsociety #mysteriousbenedictsociety #katewetherall #reyniemuldoon #constancecontraire #stickywashington #nicholasbenedict #trentonleestewart #fanart #bookart #drawing #art #sketch #illustration #bookillustration #lisbon #trainstation #youngadult #newyorktimesbestseller #ledropthacurtain #perilousjourney #adventurebooks
Benedict’s adoptive parents seeing Curtain for the first time
OK
So, i was thinking. Why is Ledroptha's name so weird but Benedict's is just Nicholas?
Well, it's mentioned in the book that their parents didn't know they were going to have twins. So it makes sense that one would be really thought out and creative, while the other would just be "oh crap it needs a name uhhhhh Nicholas" (it's thought out from a meta perspective- Nicholas means defender of the people- but in-universe not so much)
THAT MEANS THAT CURTAIN WAS THE FIRST BORN
And therefore he's older the older one while Benedict is the younger one.