Why does Gu Hongming translate 弟 as "be a good citizen" (做个好公民)?
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.