I found this explaination by John Gill very helpful and hope it sheds light on this subject.
a sin [which is] not unto death; every sin, even the least sin, is in its own nature mortal, or deserving of death; the proper wages of sin is death, yea, death eternal; yet none of the sins of God's elect are unto death, or issue in death, in fact; which is owing not to any different nature there is in their sins, or to their good works which counterbalance them; but to the grace of God, and to the blood and righteousness of Christ, by which they are pardoned and justified, and freed from obligation to punishment, or eternal death, the just demerits of them: but how should another man know that a brother's sin is not unto death, when it is of the same nature and kind with another man's? it is known by this, that he does not continue in it; he does not live in the constant commission of it; his life is not a course of iniquity; that sin he sins is not a governing one in him; though he falls into it, he rises up out of it through divine grace, and abides not in it; and he has a sense of it, and is sorry for it, after a godly sort, loaths it, and himself for it; is ashamed of it, ingenuously confesses it, and mourns over it and forsakes it: now when any strong believer or spiritual man sees or knows that a brother has sinned, and this is his case,
he shall ask; he shall pray to God for him, that he would administer comfort to him, discover his love, and apply his pardoning grace to him, and indulge him with his presence and the light of his countenance:
Brother Glen