Tough question! Let's review the historical facts, according to Scripture:
1. Satan was originally an angel (Isaiah 14:12-14)
2. Satan became prideful and wanted to take God's place (Isaiah 14:12-14)
3. God cast him out of heaven (Luke 10:18)
4. This happened before the creation of mankind (Genesis 3)
Now, let's take a stab at your questions.
1. Do angels sin? Yes. Satan was an exceedingly beautiful angel who sinned and was subsequently cast out of heaven (Ezekiel 28:12-15) by God.
2. Isn't heaven a place of no sin? Yes. By this, we mean that God cannot allow human beings that are still in their sin to enter into his presence. Angels are not human beings. They are a completely different order of being. Due to their role as special servants of God, they have access to heaven. However, like human beings, they can sin. There are good angels and evil angels (i.e. demons). Thus, when Lucifer sinned, God instantly cast him out of heaven; out of his presence forever.
[Answered by Pastor HM]
