Yes, some sins are more serious than others.
1 Jn 5:16,17 says, "If anyone sees his brother sinning, if the sin is not deadly, he should pray to God and he will give him life. This is only for those whose sin is not deadly. There is such a thing as deadly sin, about which I do not say that you should pray."
What is a "sin unto death"? What does that mean? I'm taking it to mean a sin not repented of? Or a sin which you consciously refused to repent of/refused to believe you could be forgiven for up until the end of your life?