I think the question over Jesus' peccability helps here, Paul. We traditionally, and in an important sense rightly, teach that Jesus was impeccable, that is, that He could not sin. The argument is rather simple: Jesus is God; God cannot sin; therefore Jesus cannot sin.
But there is a distinction we need to make here that we will be able to make about the blessed, too. We need to distinguish between what I might call metaphysical impeccability (MI) and practical impeccability (PI). Something is MI if to speak of something as sinning would speak of a metaphysical contradiction. God, and only God, has MI. The reason is that sin presupposes an imperfection, and God cannot be imperfect, for perfect
means (lit., by definition) "lacking nothing." So we can attribute MI to Jesus according to His divine nature. Something would be PI if even though the logical possibility of sin were present (that is, the attribution of evil to a think does not create a self-contradiction) such an outcome will simply never happen. By way of analogy, imagine I offer you a check for $100 and a check for $10,000. All things being equal, and assuming you are rational, is there any scenario in which you would choose the $10,000? Again, the key here is "all things being equal." I don't think you could never come up with a scenario in which $100 would be better than $10,000, but I do think all such scenarios would entail some inequality. The point is simple enough: a rational person will act rationally when and if they can. If they don't, it is only because there is some impediment to their doing so.
So I submit that we can also attribute PI to Jesus, here according to His human nature. Jesus was perfectly aware of right and wrong, of all possible scenarios and of every possible outcome to every action. More importantly, because of His two natures, He lived with the beatific vision in every moment of His life, which is to say, He saw God perfectly and clearly. So for Him, sin was different than it is for us. Whenever we sin, we do so because we are seeking some good and we convince ourselves (sometimes with very little work at that!) that the sin is a good way to achieve it. But Jeus saw it for what it was. And therefore, sin became an
irrational choice. As such, there is no conceivable circumstance under Jesus
would sin, because there could be no compelling reason for Him to do so. It was a
practical impossibility.
Now, I submit to you that, in heaven (and on the new earth, for that matter), we will not have MI; only God will have MI. But we will all have PI. Our natures will be restored, so we will no longer have to deal with the sin nature. And more than that, we will see God, as Paul says, clearly, face to face. In the beatific vision, we will see perfect goodness, and next to perfect goodness, even the slightest imperfection will seem the infinitely horrible loss it actually is. And thus, for us, there will be no conceivable circumstance under which we
would sin. Sin will not be a metaphysical impossibility but rather a practical one. And, if that were not enough, we will always be able to see the effects of sin by seeing those who will eternally live with that imperfection (so Isa. 66:24).
The bottom line: in the resurrection, the blessed will never again sin. Having seen God, there will never again be anything attractive in it. They will be sealed forever in their blessedness, just as the damned will be sealed forever in their rejection.
edit:
As far as angels sinning, here's a good explanation of Aquinas' argument:
http://readingthesumma.blogspot.com/201 ... ngels.html
The long and short of it is that angels saw more of God prior to their fall than we do now, but that their mistake was in trying to obtain the beatific vision (what we would think of as being "saved") on their own rather than through God's grace. In other words, some angels, much like humans today, tried to be "saved" through their works rather than through God's grace.