Families are often hard to deal with, even if you love them. This is true in real life, but it’s even more true in fiction, especially in science fiction and fantasy. A large part of the problem is that including the hero/heroine’s family in the story means that the number of characters instantly begins to proliferate: two parents, four grandparents, an unknown number of siblings, aunts, uncles, and cousins (it really isn’t plausible for the main character, both parents, and all four grandparents to have been only children, especially in an agrarian, pre-industrial, or even just pre-birth-control society). When you already have a strange world to establish and a bunch of plot-related characters to work in, the thought of making up and dealing with all those additional people (most of whom aren’t really relevant to the story you had in mind) is daunting.
Then comes the question of what to do with all those people once you have them, and how to make them individual enough so that the reader doesn’t get overwhelmed or confused. And unless the story is about the family and its relationships (which most action-adventure stories, SF, and fantasies really aren’t), one has to do all of that while developing the major characters who are actually important to the plot.
A character’s family is usually really important to the backstory and characterization, and often to the emotional plot as well, but they often aren’t that important to the action plot. In a character-centered story, this isn’t so much of a problem, but in an action-centered one, it can cause serious difficulties with balance.
One way to solve this dilemma, obviously, is to get rid of the family. This is why so many main characters are orphans (the Evil Overlord burning down the hero’s village has become an opening cliché for a good many fantasies), or adults who are estranged from their families, or who are adventuring hundreds or thousands of miles away from whatever family members they have. This works fine for a standalone or a classic trilogy that ends with awards and weddings as the validation, but these days an awful lot of things that were supposed to be standalones or trilogies end up as a series, which means that even if the main character’s family-of-origin has been disposed of by the Evil Overlord, he/she often ends up with a spouse and children long before the series winds down.
Some writers solve the problem by killing off the spouse and kids after a book or two, but one can’t do that over and over without the reader starting to wonder whether the main character is actually getting anywhere in his/her efforts to Save The World. After all, if the hero’s parents were killed by bandits and his first wife murdered by an ambitious flunky and his three kids killed by the Evil Overlord and his next girlfriend accidentally dies in an assassination attempt…well, it certainly doesn’t seem like his efforts have made the world much safer, does it?
Then there are the writers who shuffle the spouse and kids off somewhere safely offstage, so the main character can keep having adventures. This works find for one or two stories, but as a premise for an ongoing series it tends to be unsatisfying, if not downright annoying. Even a trophy wife is supposed to have some kind of presence in her husband’s life, if only “being seen in public so everybody knows he has a trophy wife.”
The third common way of dealing with the main character’s developing family is to skip ahead fifteen or twenty years and start telling stories about the next generation. Unfortunately, this puts the author right back at the beginning – what to do about the main character’s parents? – with the added problem that readers who’ve been following the series already know this character’s parents, like them a lot, and want them to continue living happy and/or interesting lives, which means that killing off the second-generation’s parents is not going to play well with those readers.
The final way of dealing with the main character’s family is to get them involved in the action plot. This works really well when the story is character-centered from the get-go (even if it’s not specifically family-centered). It also works when there’s a good fit between the characters who make up the family and the types of characters who are needed to move the action-centered plot along, but how many families are neatly made up of a hero, a thief, a swordsman, a mage, and a healer? It doesn’t work nearly as well when there are no plot-related roles other than “victim” for members of the family to occupy (realistically, how many times can a family member be mugged, kidnapped, murdered, or framed without the whole clan starting to look seriously accident-prone?).
I think the problem comes from several directions. First, some writers have trouble accepting that if they’re writing an action-centered plot, the main character’s family are going to be minor characters, no matter how important they are to the backstory and personality development of the hero/heroine. Second, the writer knows how to handle and develop major characters, but hasn’t yet figured out how to handle minor-but-important ones satisfactorily – it’s all or nothing; either fully-developed on-stage important characters or nameless spear-carriers, with no middle ground. And third, many writers have trouble juggling a large cast of characters, and adding even two parents into the equation can end up being two more than they can handle. Rather than starting to drop balls, they sensibly choose to write the extras out of the story.