of course "free will" isnt the cause of sin. its the unability of imperfect humans not to do what they freely decide. even if you decide willfully not to sin youll do it. thats the reason for sinning. imperfection. of course there is free will in heaven, because satan and all his demons willfully decided to offense god.
even in paradise earth there will be free will... but of course everyone decides not to sin there. noone will change his/her mind just like the first humans (adam and eve) and one of the first angels (satan) did.