Yes there is free will in Heaven, no we cannot sin. The way I try to explain it is this.
A perfected will has within it the capacity to choose against God if and only if an external influence leads it to sin and it chooses to give in to that temptation. Eve’s perfect will was tempted by the devil, she chose to give in and sin. The temptation did not come from within her, it was external. Mary was likely tempted throughout her life by the world around her. She chose to follow God and deny the temptation. The temptation was external to her, and she rejected it.
In Heaven, the elect are eternally separated from all evil. There is no factor there that can tempt the saved. Therefore, while the potential to sin still exists within the soul insofar as free will remains intact, there will never be a situation where that soul can be tempted, and therefore there will never be a situation where it feels inclined towards sin.
Right now we deal with concupiscence, the inbuilt inclination towards sin. That’s gone in Heaven. There will be nothing within us that wants us to sin, we will exist wholly aligned with God’s will.