Yes, it is.
I'm sorry, but you are very much mistaken.
The point is, it's perfectly legitimate to play your family as a relatively close-knit one, where an insult to one is considered an insult to all. It's also perfectly legitimate to play them as more distant relations, who don't care much about what the other family members do.
It may be legitimate but it is not historically accurate. No, of course not 'all' families were calculating and self-centered, but the great majority of them had to be. Tuchman talks about this in "A Distant Mirror." Kids spent very little time with their family until their were about 7 or 8. They were very rarely raised by their parents, who also knew next to nothing about raising children. There were books for women on things like managing your husband's estate, how to run things while under siege, and tenure law, but not on childrearing, childbirth, or anything that you might imagine could actually create bonds between a family. Peasants did that. Nobles had to fight and rule.
A 'close-knit' medieval nobles family pretty much required a set of circumstances that led to no possible conflict between them. But even if you had two fiefdoms of exactly the same size and value and both of your sons inherited one of them, you also had to have unusually non-ambitious sons, particularly in the eldest son, who may have heard of how some places give everything to the eldest son and why aren't we civilized like that? Most of the great conflicts from the Dark ages until the Renaissance were intra-family in some way, with the major exception of the Crusades having come about because Pope Urban II saw Europe being crushed under the weight of its own family squabbles and needed to both trump them with a bigger enemy while also offering an avenue to forgive the sins of anybody who had already gone about disposing of inconvenient relatives.
Of course it's possible that some family feud would trump internal family politics, but even then you'd be just as likely to see one side of the family double-dealing with the object of the feud. Relatives working against somebody who "messed with the house" could easily happen but absolutely not because "blood is thicker than water." That sentiment suggests personal bonds and affection; fighting someone who is trying to bring down your house is protecting your own interests with some natural allies. We would find your average medieval noble to be extremely cold. Finding exceptions to this is like finding a marriage that wasn't arranged. Did it happen? Sure, but hardly ever.
Whether that matters to you in BM is another question.
