Administrative Advice from Moses' Father-In-Law
It's possible that Zipporah returned to her father in Midian after the confrontation with Moses over the matter of circumcising the son she bore Moses. It is recorded that Moses sent them back, but the timing of that event is not clear. There is no account of the entire family coming out of Egypt. We do find here that Jethro now brings Moses' wife and children back to him.
Jethro also gives Moses some advice in carrying out the responsibilities of a leader among a civil nation. Just as Moses had grown tired in holding God's staff up on his own in the previous chapter, so was he wearing himself out by single-handedly dealing with all the problems of the people himself. Jethro, witnessing this, recommends that an organized leadership be put into place to handle the day-to-day issues of millions of people and animals. Remember that Jethro, a leader among the Midianites, had years of experience in leading people.
Some people have argued that such a hierarchy was against God's will. However, notice that Jethro said to institute such a captain system only if God so commanded Moses (verse 23). And it is inconceivable that Moses, who talked with God every day, would have taken such far-reaching steps without consulting with Him. Furthermore, that God sanctioned this system is clear, for He later commands that 70 elders be chosen from among those who are already "officers" over the people (Numbers 11:16)—i.e., having been declared so through the captain system.
Like chapters 15 and 16, chapter 18 also reveals that God's laws and statutes were being taught and expounded even before their formal declaration at Mount Sinai (verse 16).