Why does Gu Hongming translate 弟 as "be a good citizen" (做个好公民)? Chapter two of 學而:


  有子曰：「其為人也孝弟，而好犯上者，鮮矣；不好犯上，而好作亂者，未之有也。君子務本，本立而道生。孝弟也者，其為仁之本與！」


弟 is translated by Gu Hongming as:


  a good citizen


the footnote translates it back into Chinese as:


  做一個好公民


Legg's translation of 弟 is:


  fraternal


Is it just an extension of this fraternal duty that Gu takes to mean "a good citizen"? or what?
弟 is a 通假字, it should be 悌 nowadays.

In ancient, the number of characters is less than today. These people sometimes have to borrow the meaning from another character.   

孝: be good to your parents
悌: be good to your brothers and sisters

If one be good to your parents, brothers and sisters, the one probably would not against higher,  if he never against higher, he would not rise in rebellion.