Currently popular genealogical database programs focus primarily on the biological core family unit, often to the exclusion of other important and genealogically significant kinship relationships. I am not advocating any radical change in the way biological family units are either generally considered or recorded, but I am suggesting that there is a lot more information out there about families that is lost either because of lack of awareness or knowledge on the part of the genealogists or for the much simpler reason that there is no convenient way to record the information. In some cases, the language used by the genealogist lacks words to describe those kinship relationships.
A kinship relationship or system is defined as a system that encompasses all of the blood and marriage relationships that distinguish among different categories of kin, create rights and obligations among kin and serve as basis for the formation of certain types of kin groups.
Let me give a simple example from the Parish Register shown above from Durango, Durango, Mexico. The entries in this particular register are formulaic. Every entry contains almost exactly the same information, in the same words for each individual marriage. However, some of the changes in wording from record to record are significant and reflect various rules and traditions of the Catholic Church. Without going into an extensive analysis, I would point out that, normally, the entries always contain a reference to the "testigos" or witnesses. In most cases, these witnesses are the godparents or other relatives of one or the other of the couple getting married. If we refer back to a baptismal record for the same individuals (which I cannot do here for the couple above without extensive research) we will find that it is possible that these same "godparents" were present at the baptism. Even if the witnesses to the marriage are not godparents, they are usually related in some way and recording their presence at the marriage ceremony is important to understanding and extending the family lines.
The kinship practice of the Catholic Latin Americans of appointing godparents is called "compadrazgo." Here is a description of the practice from the Oxford University Press:
An important set of kinship practices in Roman Catholic Latin America is compadrazgo, or ritual coparenthood. The baptism of a child requires the presence of a godmother and a godfather as sponsors. By participating in this ritual, the sponsors become the ritual coparents of the child. In Latin America, godparents are expected to take an active interest in their godchildren and to help them whenever possible. However, the more important relationship is between the godparents and the parents. They become compadres ("coparents"), and they are expected to behave toward each other in new ways.The ramifications of the practice are much more extensive than covered in this brief summary and vary from country to country.
My point here is that this important kinship relationship is entirely ignored by those who do not understand it. There is no convenient way to preserve this information in the most popular genealogical database programs unless the researcher has a deep understanding of the practice and preserves the information in notes or custom fields. Even then, there is usually no way to show those relationships on the standard pedigree chart format commonly used for online family trees and extensively reproduced in paper form.
This is just one small example of what I am talking about. Even within the United States we have a variety of kinship structures that govern the way people live and die as well as govern the way they interact with their family and other relatives. Even within the standard Western European nuclear family, we have variations in kinship relationships that seriously modify the traditional model. I am in no way passing any judgment on the validity of one kinship system over another, I am merely pointing out that our conventional way of recording genealogical data ignores many (if not most) of these relationships. I would suggest that many of the puzzling and seemingly inexplicable family relationships we encounter are merely a reflection of our lack of knowledge of kinship systems.