May I trouble you to answer this question: What is the sin unto death mentioned in 1 John 5:16?
I think it is important to see that John is not discussing some particular sin. In fact, in the original, the definite article is omitted. “There is sin unto death.” That is, sometimes, as in the case of the people of Israel in the wilderness and of Moses and Aaron themselves at the rock, one sins unto death and God takes him away from this scene, but we do not need to inquire as to that when we pray for the sick. It is the Lord alone who knows whether one has sinned unto death. If so, He will take His poor, erring child home and out of the place of temptation.