Who you consider to be related is determined, to a large extent, by your cultural and social background. My Grandmother used to say, "You can choose your friends, but you can't choose your relatives." This was usually a response to something negative happening in our family. But, in a sense, you can choose your relatives. You and your family make family and many genealogical decisions based on who you think are your relatives. For example, say someone dies and they are a "relative," how close does the relationship have to be before you feel compelled to go to the funeral?
Now, this is a trick question. Because of inter-family relationships, you may refuse to go to the funeral of a member of your immediate family. But what I am talking about is not this family squabble kind of relationship, but the basic structure of your family's kinship. These relationships do not come from our knowledge of our family through contact, but are culturally established through linguistic and social mechanisms. But sometimes these relationships are established through inherited family fights and prejudices.
I frequently have people come up to me and declare that we are related. Most of the time, this relationship stems from a rather remote common ancestor. Other than acknowledging our shared ancestor, the existence of this type of relationship seldom engenders any further social contact or obligations. Occasionally, there is an incentive to share some kind of records or photos, but there is no real sense of family created merely by reason of descent from a common ancestor. The exception is belong to a remote ancestor's family organization, especially if the the remote ancestor had a large family or was famous for some reason.
In every family with which I am acquainted, there are certain family members who, through past experience, are fighting for one reason or another. In some cases the family is fragmented by coalitions of siblings. Some of this antagonism spills down through the generations and their descendants lose contact with each other over the years. My maternal grandparents' family was a good example of this issue. As children, we had very good relationships with my Mother's parents but virtually no contact at all with some of my Mother's siblings. In fact, my contact with some of my uncles and aunts was limited to one or two visits during their lifetime while others were visited frequently. Over the years, I have had no contact at all with the cousins who are the children of these estranged relatives but have more frequent contact with other cousins of family members who we visited. I can only guess at the underlying causes of this disruption in the family. Interestingly, I see the same type of thing happening in my own family with my own siblings.
It appears, that to some extent, inter-familial relationships can govern who we consider to be our relatives. Of course, from a genealogical sense, anyone who is related by blood or marriage is a candidate for research, but I suggest that we select our research efforts, in some instances, based on our concept of who we are willing to associate with. I have found this to be the case with some of my friends who were raised by their mothers when their fathers had abandoned the home and there was a divorce. There is a measurable degree of antagonism in researching the divorced husband's line.
One of the common occurrences in families, the re-marriage of a spouse after a death or divorce, creates another interesting conflict situation. This seems to happen frequently when a mother or father remarries when the children of the first (or second or whatever) marriage are adults with children of their own. There are frequent instances where the parent's new spouse is unwelcome and literally written out of the family record. I saw this in my own family with my paternal grandfather's second wife. Although she was technically my grandmother and lived until I was quite old, I saw her on only one occasion.
Maybe we need to examine our attitude towards our present and past family members to see if we are making genealogical research decisions based on inherited family fights.
In all of this, there is another underlying issue; that is, the relationship we have with the families of those relatives we call "In-laws." This issue also extends to the spouses of our bloodline relatives. Are you really related to the wife of your bloodline uncle or aunt, the children of a grandparent? Usually, you are not related to your own spouse "except by marriage" and so this relationship is a cause of a certain amount of discussion among genealogists as to the appropriateness of "doing research" for unrelated lines. Do you feel any obligation at all to research the family of those who are married to your brothers and sisters?
Again, maybe we need to examine our attitudes towards those who "married into" the family and evaluate whether or not we consider their families as proper subjects of our research. When we read a surname book or look at research done by relatives, it is useful to recognize that family relationships may have affected the structure and content of those records.