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The Answer - Meaning of In-law
The term "step" did not come into 'common' usage until the late-1850s and even then it took awhile to really catch on as a relationship description. By the time of the 1870 census most true "step" relationships were being recorded as such. There were exceptions, though.
Prior to that time, the term "in-law" was routinely used to signify a child who was the biological child of one partner only. Thus, if a man had children from a previous marriage then his children would have been known as the wife's child"-in-law" - signifying that the child had become part of the wife's family by the operation of law.
Herein also lies a difficulty when describing what we know as an "in-law" today.
The man or woman who married into a family and would be known to us, today, as a daughter or son-in-law, was treated as a part of the whole family unit and therefor, was simply referred to as a "son" or "daughter". If the biological child predeceased his/her spouse and/or parents, the son or daughter-in-law would still be referred to as a "son" or "daughter". Consequently, great care must be exercised whenever a relationship definition is encountered in a document, particularly in a Will.
These unfamiliar definitions of relationships have to be treated with caution while we examine census. However, the biggest problem can lay with Wills, land records and other documents where the whole household inter-relationships are not specified.