Invented by the church to sell a magic cure designed to keep people fearful and tied to religion. Old testament never said we inherited sin, just the opposite. A child will not inherit the sins of his father nor will the father inherit the sins of the son several times it says so
The very definitions of the words 'right' and 'wrong' are created by mankind. If we had decreed vice to be 'right', to be commonly accepted in the society, then it wouldn't be counted as a sin, would it? The definition of 'sin' varies from person to person: some might consider murder a sin, but what if it is done in self-defense? What if it had been a kill-or-be-killed situation? Is it still a 'sin' then?
In my opinion, sins were created by a group of people in order to control others with fear. Nothing in this world is truly fact, everything is theory. Humans are power hungry and by creating a system in which people feel that they need to follow these rules in order to get into heaven, it makes it easy for the people in charge to control others.
What else would it be? Of course sin is a man-made concept. It isn't a law of physics. Sin is an explanation and/or justification of evil in this world. Sin means one thing to one individual and something else to another. There is no universal sin. It's just a concept that exists in the mind of man.
Of course sin is a man made concept. If it was not, then it wouldn't be as changeable as it is. How many things were considered a sin years ago that are perfectly OK now? I don't even know if the number is countable at the moment. If it wasn't a man made concept, sins would not change.
The concept of "Sin" is indeed a man-made notion symbolically describing things that the humans of the time agreed were so wrong as to warrant the creating of a new word. Even if we didn't have the word sin as a signifier, we would still know deep down that certain things are wrong in their nature: they are done with an intent to do harm (psychological), Hurt (physical), are criminal in nature, or malicious in intent.
Morals are something that are ingrained in our society as a consequence of our past. We have formed what is acceptable or not and defined the good actions as moral. But, just because it we have defined it does not mean it is not inherent in us. Thus, if morals are inherent and sins are the opposition, sins are therefore not manmade concepts but the opposition to our ingrained moral obligation.