Great houses of powerful families stand in the heart of many epics, such as the Atreides/Harkonnens of Dune, and the Lannisters of “A Song of Ice and Fire.” Remnants of families reunite in LOTR and are well known in the Harry Potter series.
Though powerful families are not the center of my novellas, instead Shevata's family is a "non-family" given her breeding for the purpose of war. However, the use of long lines of relatives enriches the stories, particularly in epics with large casts of well-developed characters leading to intrigue, action, tragedy, and victory.
Families are usually opposed to each other for a variety of reasons, and often, if royal, use marriage to arrive at a consensual agreement to stave off war. Some families are the “good guys” like the Weasleys in Harry Potter, the Atreides in Dune, and the Starks in A Song of Ice and Fire, which coincide with the Malfoys, the Harkonnens, and the Lannisters, respectively, as the “bad guys.”
Most of the fantasy families are not poor (exception the Weasleys, more middle-class), have some kind of political influence, and have their own brand of corruption and relationships. In fantasy, most of them look alike, usually hair color, and when well written, they act enough alike to be consistent but different enough to be stand alone characters.
The backgrounds of dark fantasy characters have almost as much to do with their development as their actions. The tyrannical or benevolent patriarch, the offspring competitions for power, or the husband/wife marriage of love or politics make for excellent backgrounds in epic stories.
New authors can get inspiration from fictional families, as well as historical family lines and even our own family units. Every person comes from a family, and defining the family is defining the character.