A large part of Moll’s story is all about her marriages to different men, and how those marriages carry out over time. All of her marriages seemed plausible until she comes to the realization that she has in fact married her brother, and that her mother-in-law is in reality her birth mother. This situation was not expected whatsoever, and once Moll’s revelation about the truth of the situation occurs, it is hard not to stop and try to take in everything that just took place. Being an 18th century novel, much of what the story is focused on is the plausibility of situations that characters in their respective novels are put in. The plausibility of happening upon your brother, from your birth mother, whom you never knew anything about, and eventually marrying him, does not seem in the least bit plausible. When Moll reveals to him that he married his sister she “saw him turn pale, and look wild,” just as any person who might have found they married their sister would react. It is possible Defoe included the incident to show the continuous misfortune of Moll, and her bad luck in finding a suitable husband, especially in a society where being married was such an important aspect of life.
Moll Flanders is a work of fiction, but given the time period in which it was written, it seems out of place to have an event so unlikely to happen, actually take place.