Family in law is a phrase that was used in America during the 1700s. In those days, most Americans were farmers and many had a large extended family. People would marry and have many children.
In order to raise a large number of children, families would share one house. As time went on, more and more people started to move away from rural areas and lived in cities. As a result, they were no longer living with all of their relatives and this created problems for the family.
When two families live under one roof, the problems associated with having too many people living under one roof are multiplied. That’s why in-law was used to referring to anyone who wasn’t related to the family’s FAMILY LAW husband. If someone is related to the wife, he or she is called a brother, sister, cousin, etc., but if he or she isn’t, he or she is called an in-law.
Today, in-laws are usually referred to as relatives that have married into a family. The same term is also used for people who married into a family after their parents and grandparents have died. In today’s terms, in-laws would be called step-relatives. If you look at the word “step,” you will realize that it means to step up.