So, in heaven, it’s impossible to sin.
Do those in heaven have the ability to discern between x being God’s will, and x being opposed to God’s will?
So, if they have the ability to recognize God’s will, then asking for something they specifically know is contrary to God’s will is a sin. On earth, we are very limited in how we perceive things. But in heaven, there’s just reality… no excuses, no explanations, no pretending. So if a saint is invoked, I would be surprised if God didn’t allow them some sort of understanding of the situation they’re being invoked into.
Otherwise, we’re left with a mental image of the saints being parrots.
“Hey, Mom, ask Dad if I can have a .243 Winchester for Christmas…”
(robot voice) “Dear, shouldn’t we get Junior a .243 Winchester for Christmas?”
Versus, “Dear, Junior wants a .243 Winchester for Christmas… I don’t think he’s really mature enough for that, but what do you think would be a good alternative for him? Whatever you think is best.”
We ask for stuff that can hurt us. Or is wrong for us. Or that we’re not equipped to handle. Or things that aren’t meant for us. Or things that God wishes to give us— but not quite yet. And so ultimately, when our prayers mature, the most successful ones end up being along the lines of, “Thy will be done.”
I would suspect that the prayers of those in heaven align similarly-- where they’re not so concerned with God giving us the exact details of what we desire, a la Santa Claus, but giving us what we need the most, in his plan for us.