V.1-5 The land from which they had come (Egypt) and the land to which they were going (Canaan) both had very bad and immoral customs. The contrast was to do what God told them to do, because He was the Lord and therefore perfect. Verse 5 was the glorious motive and result, if they obeyed. We too!
V.6 This verse is the key to understanding the chapter — we must not mix the intimacies of marriage with family relationships. Satan has driven people into doing these things. In 1 Corinthians 5:1, 3 we see that believers are capable of doing these same evils.