yes, we know that the Catholic church teaches a lot of add-ons to Scripture.
But God still doesn’t differentiate between “mortal” sins and “venial” sins.
All sin is violation of God’s command and is worthy of death.
If you break one, you are guilty of all, according to James, the RCC’s favorite author of the NT.
[1 John 5:16] If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.
[17] All unrighteousness is sin: and there is a sin not unto death.
My quotation isn't to agree with any particular interpretation of what a "sin unto death" and "a sin not unto death" are. I'm just noticing that unless there's considerable error in the Bibles that most Christians use, the Bible makes a distinction between the two, even though I've never seen a scriptural delineation of which sins fall in which category. Accordingly, Scripture itself seems to disprove the insistence that all sin is sin without any sort of distinction soever: "all unrighteousness is sin," but not all sin is the same, or else the surrounding words wouldn't make sense. (I don't view James 2:10-11 and similar passages as a contradiction, and for deeper reasons than some a priori belief that Scripture contains no contradictions.)