Why do you think other atheists don't care about what's moral? Because that's all sin is, right, an immoral act.

Can you tell me what is the difference between sin and an immoral act?

Of course sin and morality are both subjective and based on the values of the person, community or culture talking about it. Sin is not more objective by any means. It is just what different cultures and cults see as immoral. If sin was objective, religions wouldn't disagree on what their imaginary gods wanted from us.