all sin IS evil, so start by telling them you agree!

…
Catholics believe that, and do not water down venial sins as “acceptable”!
In response to your Protestant friends, just quote scripture…
1 John 5:16-17 (RSV): “If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. All wrongdoing is sin, but there is a sin which is not mortal.”
Luke 12:47-48: “And that servant who knew his master’s will, but did not make ready or act according to his will, shall receive a severe beating. But he who did not know, and did what deserved a beating, shall receive a light beating. Every one to whom much is given, of him will much be required; and of him to whom men commit much they will demand the more.” (cf. Lev. 5:17, Lk. 23:34)
John 19:11: “’. . .** he who delivered me to you has the greater sin.**’”
The Bible also refers to (mortal) sins which - if not repented of - will exclude one from heaven (1 Cor. 6:9-10; Gal. 1:8; Eph. 5:5; Heb. 12:16; Rev. 22:15). The list does not include children stealing cookies while mom’s not looking, or adults who have 4 glasses of wine when they should have stopped at two (assuming that they have done no more harm than become a bit tipsy).
There’s a good discussion
in this blog