Angels who have not sinned from the beginning and are in heaven will not sin because they were elected by God not to fall into sin (1 Timothy 5:21).  Heavenly angels are ministering spirits who carry out the will of God (Psalm 103:20; Matt. 6:10), and they work against the fallen angels (demonic horde) who in turn work against God.  Therefore, it seems that from the beginning, before the foundation of the world, the angelic realm made its decision; and one-third of the angels sided with Satan and rebelled against God (Revelation 12:4).  Furthermore, we see no indication in Scripture that there will be a future rebellion of the angels who are now servants of God.

Finally, the presence of holiness, God’s holiness, is a factor that cannot be overlooked.  I am sure that these beings are, to some degree, experiencing the incredible holiness and purity of the Lord God and have also encountered the hideous evil of the fallen angels.  It would make sense to say that, given such awareness and experience and the fact they were elected not to fall, those angels who are presently with God will never rebel against him.

Thus, perhaps for similar reasons that, after the final resurrection unto eternal life, heavenly men and women will no longer sin, it seems that heavenly angels exist in a state where they will not sin.