My definition of "sin" is "an act toward another, that they disapprove of, or that causes them some form of injury". If we take this definition as (at least somewhat) correct, then Saint's question has real merit (as inaction, that causes someone else to suffer, would be a sin).
My answer, however, is that to directly act against someone is worse then allowing injury to occur to someone. I say this because, in the latter case, I didn't do anything wrong, but I didn't do anything right either.
That's an interesting moral question, but it is really dependant upon a person's beliefs. Some religions believe that it is sin for a woman to show her face or any part of her body. Some believe it's sin to eat pork.
Sins are just guidelines for what is right and wrong. It's a basic outline of morality. And it really depends upon the society you live in, and the beliefs you follow.
"Moral wrong" and "Evil/Sin" are two diffrent concepts, depending on your definition.
Morally right or wrong is a purely social concept. Morallity does not need to apply to a religious concept like Sin and Evil.
Sin and Evil, in a religious context, is simply the sate that we as flesh abiding humans are constantly in. It has nothing to do with morals. So, in essencem there is no such thing as a greater or lesser sin. we are sin. if your definition of sin is that which is against God (which would be Gods definition of sin).
Moral evil need not apply to religion, as we cannot really put morals onto Gods actions. So, your original post would be more of a moral question, not a sin or evil question.