It is very important to understand the context of this verse. In the two preceding verses, John has talked about the assurance that believers' prayers will be heard and answered. Then in verse 16 he applies that promise to the specific case of prayer in another's behalf. In doing so, he discusses two classes of sin--one in which there is hope for the sinner, and another in which there is no hope.
In the first case, prayer could lead to the sinner's recovery and redemption, but the second situation held no guarantee that prayer would bring salvation. It seems quite apparent that the "sin unto death" is referring to the unpardonable sin. Those who are recognised as rejecters of the Spirit and are hardened by continual transgression would not be open to further conviction of their sin. In such a situation, the positive assurances of verses 14 and 15 could not be applied.
For all other cases, excepting the unpardonable sin, God will give life to the one who is being prayed for---contingent, of course, upon that person's repentance and acceptance of Christ.