You have to go to the source
You have to remember Jesus gave the Keys to Heaven, which is the power to say what sins are held and what are not, and what you have to believe to be part of the Body of Christ, to Saint Peter. Only Saint Peter can define what faith and morals. "Peter" was the first papal name, given to Simon.
And when you go to the Ex Cathedra from the Chair of Saint Peter, the infallible and indefectable teachings of the Church, the sins are listed. Avoiding sin and the 9 ways to avoid taking part in someone else's sin, seems quite do-able.
Others catechisms include avoidable occasions of sin, both proximate and remote as sins. I'm looking at you Saint Thomas Aquinas. For example, Saint Aquinas says even a mere thought of sex is a mortal sin and that it is even possible to commit sin against no one but yourself! And Saints who are doctors of the Church are right, right? Not always! The Church teaches that Saints like Saint Aquinas are right unless the Pope says something else.
I'll see you a saint, and raise you a Pope
Saint Bellarmine's catechism says that a sin is against someone else; either GOD (first three commandments) or another person (the remaining commandments). Pope Saint Pius X echoes this and says that occasions of sin should be avoided. I think there is also a difference in how they describe an impure thought: e.g. King David plotting to kill Uriah so he could have sex with Bathsheba is an impure Thought. As the Lord said, "He that looks at a woman to lust after her commits adultery". IIRC, (debate me) Pope Saint Pius X also said that the kings who wanted to marry Sarah committed no sin because they didn't know she was Abraham's wife. Thus, wanting to have sex and raise kids with a woman is not, in and of itself a sin if you have good reason e.g. Abraham's lie, to believe she is not married. Saint Thomas Aquinas leaves me wondering how do people even get married since he seems to forbid thinking about it. I think I understand it now.