Really to sanctify something is to cleanse it. It's not the case that something is more or less holy. It is more a case of something is either holy or unholy, clean or unclean, good or evil. In the case of sins this is where the proscriptions of the Law reveal a gradient in the sins it would seem on the surface, but the idea prior to Christ was not so much to say certain sins are more unholy or not, but rather to cleanse them took different methods, ergo to sanctify the person or the nation back to God certain sins and their effects could only be cleansed through one of these three mediums; water, fire, or blood.
So in the Old Testament certain sins like eating unclean animals only required water and then a period of seclusion to cleanse and restore the person, to seal vows one would sanctify their vows either starting or completing them by fire via a burnt offering sacrifce, and then some sins indeed required blood, the mortal sins like adultery and idolatry that required in order to sanctify the people and the land back to God that they must need to kill the sinner.
In the New Testament, though the Law is not abolished, it's the Spirit behind the Law that is more important. The essentials of sanctification as in the Law is still the same by water, blood, and fire; but now that much of the Law is fulfilled through Jesus the process takes meanings tied to Jesus and God's offer of forgiveness. The baptism of Jesus and the rite of baptism put forward by John the Baptist are the sanctification of water. Jesus crucifixion and serving as the Lamb substituted for the sinners worthy of death that was sacrificed for the world and belief on his holy blood being the sanctification of blood. The apostles receiving the Holy Spirit in book of Acts shortly after Jesus' resurrection and ascension, as well as the prophecies of the end of the world by melting down such as in the Apocalypse representing the sanctification by fire.