Well for starters - from my reading of the plot synopsis - two of the main characters are illegal immigrants, this puts them in transgression of the law, but justice dictates that they have a right to pursue a happer life in America
(better living conditions), and, in Eddie's case to provide for his family. Try and look for instances in the novel where something unfair (unjust) is happening, which isn't actually against the law.
It is also important when responding this question to not only address the 'articulates the notion that it is very different from statutory law' part, but also the 'illustrates the importance of justice' part as well. What events in the book illustrate the importance of justice?
If you are still having difficulty, it might help to think about what actions make us perceive certain characters in the play as being reprehensible, selfish or corrupt; and also what the characters have to say on the subject of justice and the law
(the lawyer, I am guessing probably has quite a bit to say with respect to this for instance, hint, hint).