“A word, deed or desire in opposition to the eternal law” -St. Augustine
sin is sin no matter how society changes, because sin is without God, and God can have no part in sin, and that cannot change because God cannot change, God tells us Himself - *“I am the Lord, I change not.” Malachi 3:6 *
sin is committed whenever a person knows that something is contrary to the law of God and then freely does the action anyway,
now what makes a sin mortally sinful? well, God has revealed to us the sins most displeasing to Him in the ten commandments, although even before God gave the ten commandments to Moses, His commandments were known by other means, like for example (although more obvious) when God cursed Kane after he killed his brother Able, or when God destroyed Sodom and Gomorrah and the neighboring cities for their grievous sins against nature,
whatever is not of God, is without God, and is sinful, but whether it is mortal or venial would depend on your knowledge and intent regarding sin and Gods laws,
in which case i am obligated to tell you, that you are morally obligated to learn these things through the Catholic Church, you are morally obligated to the best of your ability, to seek the teachings of God through the Church, and put them in to practice, your virtues, the commandments, and other obligatory and morally necessary things,
although i’m pretty sure your asking, what makes something more evil than something else? the answer to that is, it depends on your knowledge and intent(as i said before) and although i’m sure there is a more deep and specific answer to that, perhaps by one of our great theologians or doctors of the church, that is as much as i can explain it personally.
although to get more specific, there are matters of tradition, such as fasting and penance, which the church can change, but in regards to morality and other sins revealed by God, those cannot be changed.
and although depending on your knowledge and intent something may not be mortally sinful, but no matter what, if you commit a sin even unknowingly, it is still a venial sin.
if you want to understand how God plays a part in our world without having any part in sin, i’d suggest reading this -
it’s a small book, and easy to understand.
ok i hope this helps, tc