It has to do with how we define "new revelation". There were many religious literature works in the first century, alongside the Gospels and Epistles. Yet the Holy Spirit made sure to preserve only four of the many gospels, and the epistles that we have in the NT. There were other written works that contained much truth, but the Holy Spirit chose not to include them in what the church now recognises as the canon of Scripture. However, we can learn a lot from these Apographal works. I have a whole library of Christian books, but none of them are Scripture, yet I enjoy reading them and sense the Holy Spirit speaking to me through them.
The type of new revelation that I would not accept are the ones that come from church tradition, such as the position of Mary in the scheme of things, or like some in the Thessalonian church teaching that the resurrection had happened already. Also the "new" revelations that have come out of the Prosperity doctrine that are found nowhere in the Bible. I really don't know of any other definition of "new" revelation that would not be attempts to add something new to the Scriptures.
My definition of "ex-Biblical" is prophecy or words of knowledge, wisdom, discernment of spirits, that are not expressly quoted in the Bible, but are totally consistent with what is already written there, and with the nature and character of God.
I think we have to be very careful when a word of knowledge has been revealed that could cause a major upset in the church. There has to be much wisdom on how to share the word so as to have the least impact on the fellowship as a whole. Blurting out someone's sin in public could cause great harm and alienate a brother or sister from the fellowship and perhaps from Christ Himself. If the word is wrong, then the person giving it could face legal action for defamation of character. If a person is involved in the type of sin that you described, then sharing it like: "It is important that you ensure that you are ruling your family well and that your children are living holy lives before the Lord, so as to protect your pastoral status. One of the important qualifications for an elder (aka pastor) is that he is able to rule his family well. Any family member seriously falling below the standard of holiness could disqualify you as pastor and you may be required to resign your position." In this way, you are not directly accusing anyone, but the pastor may take the warning and check his son out to ensure that he is not doing anything immoral. The pastor then has several choices: either the son stops the immoral conduct or leave the family home, or the pastor steps down until his domestic situation is resolved. This is much better than telling the pastor that his son is sleeping with a female church member. The whole point of the word of knowledge is to protect the pastor, enable the problem to be dealt with discretely, and seeking to restore both the son and the church member to holiness, and to prevent a major division in the church.
Often a word of knowledge is given to someone in order to give wisdom on how to pray for the situation, and to sort out with the Lord about the best way to share it with the person involved without blowing everything wide open. What causes division in a fellowship over sin is a lack of wisdom about how the sin is dealt with. One can use a sledge hammer to crack a walnut and that destroys the walnut making it useless for anything else.
