Well Luke 10:18 states And He said to them, “I saw Satan fall like lightning from heaven.
So, I am not sure as to any proof or reference that God "cast" Satan from Heaven.
Another interesting thing to note is the reference to the book of Job, chapter 1, verse 6 that states; Now there was a day when the sons of God came to present themselves before the Lord, and Satan also came among them.
This to me implies that Satan is a Son of God and presents himself as one. Having been the highest angel of Heaven and keeping in mind that God co-exists without time and with time, Satan still then presents himself to God as a son of God.
In regards to the actual question of sin in Heaven we can deduce that if God is all as stated in Proverbs 15:3 The eyes of the Lord are in every place, keeping watch on the evil and the good. and again in Jeremiah 23:23 “Am I a God near at hand,” says the Lord,
“And not a God afar off? 24 Can anyone hide himself in secret places, So I shall not see him?” says the Lord; “Do I not fill heaven and earth?” says the Lord. Also in Daniel 2:22 He reveals deep and secret things; He knows what is in the darkness, and light dwells with Him. Im sure there are more but this one stand out the most to me Psalm 139:8 If I ascend into heaven, You are there; If I make my bed in hell, behold, You are there.
God is all, none and everywhere without being anywhere. He is so far beyond our understanding and perfect beyond measure so that its possible the concept of sin does not apply to Him. Who are we to say that God cannot do something? Is not everything created by Him? To include Satan? And who are we as mere men that we can understand the ways of the Lord? David says that if he makes his bed in hell, God is there. Also Heaven is brought down to earth (Revelation 21:2 Then I, John,[a] saw the holy city, New Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband.) so therefore it is not we as believers that go to Heaven but Heaven that comes down to us when all is restored and made new and the old washes away.
The concept of sin, in my opinion being but a man, is anything that makes us turn away from serving God as the only One and perfect Lord. So sin can exist anywhere, at anytime if we turn from serving the Yaweh.