"If anyone sees his brother committing a sin not leading to death, he shall ask and God will for him give life to those who commit sin not leading to death. There is a sin leading to death; I do not say that he should make request for this."--1 John 5:16. The key phrases in this verse are the sin leading to death and the sin not leading to death. According to what I have learned during my time here at the Bear Valley Bible Institute of Denver is that the sin leading to death is the sin that has not been repented of. The sin not leading to death on the other hand is the sin that has been repented of.
That being said, the concept in 1 John 5:16, in light of the context is pretty simple and straightforward. The context of this passage tells us that if we ask something of God according to His will, He hears us. All we do must be according to God's will. We must seek His will. The broader context speaks of those that believe in Christ. So if I understand this verse correctly, if we ask forgiveness for a sin our brother is committing, God will forgive him on the condition that the brother (or sister) repents. On a side note, when Jesus said "Father, forgive them for they know not what they do.", the conditions were still the same, they had to repent. Forgiveness does not come without repentance.
1 John 5 is the final chapter in 1 John and ends so beautifully. It says in 1 John 5:20-21 that the Son of God came to give us understanding to know Him who is true and we as Christians are in Him who is true. The final verse tells us to guard ourselves from idols. We do this by obeying His commands.
DO SOMETHING ABOUT IT! PRAY ABOUT IT! LOVE Y'ALL! GOD BLESS!