I think you’re reading too literally into the words sin and charity. Sin in this case just means a transgression or wrong to you or your community. Charity here is used in the philosophical sense, meaning an openness to tolerating transgressions.

For forgiveness to mean anything at all you need to forgive an actual injury. “Forgiving” someone for doing what you don’t mind them doing anyway is not forgiveness, it’s tolerance.

So what, then, is an unforgivable crime? You have no choice; you must tolerate everything or go insane.

There's no "must" here. Forgiveness is hard. That's why it's a virtue.

It's not without its benefits though. Forgiving someone who grievously hurt you or your family is the first step to letting go and moving on with your life.