One issue frequently brought up against Christianity is that if there is a God, and Heaven is what we understand it to be, then the idea starts to not make much sense. If we cannot commit sin in Heaven, then what's the point of this life? If He's going to take away our free will in the end, then why bother creating us with it in the first place?
I don't understand why this is an issue at all. In real life we have laws that must be followed or we get fined, imprisoned, or possibly put to death. Does this have any effect on our free will? If there is a penalty for murdering someone, then does that mean I cannot possibly murder someone? I am just as capable of murder as I would be if it were legal. The fact that there are consequences for our actions does not mean we are not free to choose those actions.
In other words, it is possible to sin in Heaven. A big part of the reason we're here in the first place is so that we can learn to live a life in accordance with the gospel. So that we can learn to choose not to sin. It's the reason some people won't be able to enter into Heaven, because they'd just screw it all up. It's why it's important to become a disciple rather than simply do what disciples do.