Except for the orphaned farmboy of fantasy cliche, every character has family. (Even the orphaned farmboy has family, they're just deceased / missing / secretly evil. Sometimes all of the above, improbably.) The rootless character is a popular one in fantasy - even those who have living family and relatives may not speak or see them in the story's timeline. Even in these cases, though, the family - real or adopted - leaves a mark on the character, influencing their background and personality.
Other tales either involve the family on the fringes of the plot or sometimes, right in the center of it. To be honest, this is usually the kind of book I prefer. I'm fascinated by the interplay of family ties, loyalty to blood relatives versus loyalty to found family, the lengths people will go to protect a family member ... and the clashes of personality. After all, as the saying goes, you can't pick your family, so what do you do when a family member is someone you wouldn't choose to have tea with?
Or worse, a criminal, murderer, antagonist? This in particular is something I've always been intrigued with, from the very start: my first two novel attempts featured characters who were closely related to the main villain. I'm still playing with the idea. To me, it's less interesting as a shock reveal than as something learned earlier in the plot, a dilemma to wrestle with.
As an only child, I have always been an outside observer to the interplay of siblings. My cousins all live(d) a considerable distance away, so I wasn't regularly exposed to that relationship, either. Perhaps that's the reason I so enjoy writing about siblings ... and no one's told me that I've gotten them terribly wrong (yet), so I must have absorbed something from watching everyone else's.
Of course, having been homeschooled gave me a slightly different perspective on siblings, too. I think there's a tendency when you're young to make friends with your closest age peer and then write off their brothers and sisters as annoying pests. I remember very distinctly suddenly learning that my friend's siblings were actually a lot of fun, and that stayed with me.
What about characters who are already married or who have children? Seems the married characters typically only show up when it's unhappy or troubled, or in sequels where the romance was played out in a previous volume. And unless the child is a catalyst to the plot - abducted, parent is trying to make a better life for them and that's the primary storyline, etc - you don't often see them, either. At least, not in the books I've read, though of course, I can think of exceptions.
... and there should be more of them. Obviously, the absence of such characters is partly due to the fact that it's hard to "work around" them in a plot: the related character is tied down, constricted, unlikely to be able to make too many moves without considering their spouse / child. But isn't that part of the fun for the writer? The puzzle of making that conflict an integral part of the story?
It obviously works better for some kinds of books - tales that are more intimate, character focused, or political, rather than sweeping quest sagas or war novels. But in the end, we all have family.