1 Samuel 12:23 (NKJV)
Moreover, as for me, far be it from me that I should sin against the Lord in ceasing to pray for you; but I will teach you the good and the right way.
Notice that it is not a sin against the one whom I failed to pray for, rather it is a sin against the LORD. That is the way all sin is. It is God who is offended by our sin.
Notice the consequences too, in ceasing to pray for others, the implication is that the intercession will stop and I (the praying person) will not teach the other person the "good and right way." Who does the prayer change first? The one praying. How is he changed? He helps others, then they are changed. This is how the will of God works in prayer. It changes me first and then I change others.
Psalm 51:4a (NKJV)
Against You, You only, have I sinned