No, I think the idea of absolute morals, generally agreed upon by the people or those in charge, is what determines justice.
Are you saying something like:
There is no sort of independently existent "justice", what "justice" is (or what determines justice) is a sort of precipitate from the coming together of a large number of people with moral preferences the validity of which is indeterminate, indeterminable, or irrelevant? I say "precipitate" in an effort to avoid stating one way, like consensus or polling, that the justice thing, whatever it is, is determined.
Another way to ask my question is, When a kid says, "Hey, no fair!" is there anything meaningful which distinguishes that statement from "Hey, I don't like that!"
(Gotta go now, Stuff to do.)