what are we supposed to say again when christians ask us where we get our morality?

[Whateverist]

Better to say that prosocial behavioral dispositions evolved because they had survival value. Does that make them moral? No. Does it underpin what we describe as moral. Most likely. Morality is more like beauty than it is like reality. There is no objective basis for morality.

Perhaps. Morality has practical applications that extend into personal and public life much differently than aesthetics. It sounds like you put morality in the category of preference rather than proprietary. If so then the concept of justice has no foundation and I hate to say this because I know it peeves you but that's moral nihilism.

I only know "justice" as an attempt by people to use collective force to impose agreed upon norms on themselves. I don't think there is a Platonic perfect "justice" floating out beyond the ether which the attempts of men fall short of or approach to some degree. In any attempt to codify fair play, there are likely to be some who lose out in some way. I'm not at all sure that there is always a perfect path to justice. There may always be winners and losers in the best of all possible worlds.