In an ensemble show, there's generally at least one character whose family history we don't know much about. Often this is something that the other characters and the audience don't take much notice of, assuming that not everyone's family makes for good television.
Sometimes this is the case. But sometimes it's a setup for the Prodigal Family.
Family showing up out of nowhere is an inconvenience in the real world, to be certain, but on television your uninvited relatives are people you've spent your entire adult life trying to escape. It's worked so far, but now they're here and you (and everyone around you) have to deal with them.
Prodigal families come in two major types: amicable and possessive. Amicable families are generally more embarrassing than anything, and tend to share reputation-destroying secrets with the characters' friends and enemies. Their motives are benign and mundane: They want to keep Character abreast of a family situation, they want to stay in touch, sometimes they just happen to be in town and want to talk. They're often unaware of the animosity Character has for them, or if they do recognize it they don't much care. They rarely intend to interact with Character for longer than a few days before moving on. Episodes that feature an amicable Prodigal Family tend to end with Character showing some sign that she still loves them, no matter how much they embarrass and inconvenience her.
Possessive families are more extreme. They've been actively hunting for Character for some time, and they want her back. Often evil, but sometimes have Character's best interests at heart without knowing what's really best for her. An encounter with a possessive family generally ends in either a showdown between the family and Character's friends (if the family is evil) or a painful decision by Character to stay on the show (if they're just misguided). Either way, friendship plays an important part in the conclusion and the arc ends with An Aesop about the true meaning of family, i.e., not those to whom you're born, but those to whom you belong.
- In Sarah Caudwell's The Sirens Sang of Murder, Cantrip's uncle Colonel Cantrip, whose is occasionally shipped off to London for Cantrip to look after. (He was first mentioned in Thus Was Adonis Murdered, when Cantrip needed to con a retired soldier and planned to mention his uncle in doing so. Cantrip figured there was little hope that the suspect hadn't heard of him.) The Colonel is an elderly retired soldier who belongs to a club that was founded by military veterans whose behaviour was so outrageous that no other club would have them. (The club staff don't turn a hair when the Colonel fires a gun to prove that yes, he really was threatening someone with a loaded weapon.)
- In Lindsey Davis' Marcus Didius Falco novels, Marcus' entire family tree, highlighted by the annotations on the family tree. We meet several of them in Poseidon's Gold, including his disreputable auctioneer father, who left his mother years ago; in Scandal Takes a Holiday, we meet the uncle "nobody ever talks about", his mother's eldest brother.
- In Margaret Frazer's The Novice's Tale, Sister Thomasine's aunt Ermintrude, who shows up at the convent's guesthall periodically with her staff and her objectionable pets (e.g. monkeys), and keeps offering to take the shy, devout Thomasine out of there and find her a vigorous husband.
- In Diana Wynne Jones's Howl's Moving Castle, Howl is this to his sister Megan - dropping in at odd intervals, spoiling her kids, and not settling down with a visible respectable job.
- Happens in Black Books when Manny's incredibly irritating parents show up for the weekend.
- Buffy the Vampire Slayer: Tara's relatives are a textbook evil possessive family. They've been telling her she's a demon all her life with the intent of keeping her "down on the farm," and they never expected her to escape — or to hook up with the one group most likely to be able to disprove them (Until season six, anyway).
- Subverted on Angel, when Fred's parents show up. Fred runs and hides, and everyone assumes she has some reason to be afraid of them. Turns out Fred isn't running from them, but from having to face that if she hasn't seen them in five years, then she really did spend five years in hell. Her parents are in fact probably the kindest, most supportive parents in the entire Buffyverse.
- Night Court's resident Casanova Wannabe Dan Fielding prided himself on his suave, urbane image; an unexpected visit from his parents collapsed the lies he'd been feeding his co-workers and friends, revealing his bayou-country background and (in Dan's mind, at least) the most embarrassing first name ever.
- On Star Trek: Deep Space Nine, Ezri's family only cared about money (Ezri revealed she had spent her life trying to escape them). The only exception is her sensitive, artistic little brother, who is desperate for any familial love at all — and winds up committing a murder that he thinks will make them respect him.
- 30 Rock: Jack's entire family basically fits this trope to a tee. His mother is a massive inconvenience, his father and brother are con men, and the rest are various drunks and good old Irish Catholic fighters. Obviously, he is less than pleased when they show up on his doorstep.
- Newhart; George's obnoxious cousin from Milwaukee shows up and annoys the whole town to the point where George has to stand between him and an angry mob. The cousin then says something rude to George who punches him out.
- Jameson's family in Girls With Slingshots turn up at his wedding and reveal that they are clowns. Literally, white makeup, red ball noses, floppy shoes and all. Jameson's deep shame at his heritage comes to the fore but the gang assure him it's nothing to be ashamed of.
- Daria's Jane Lane's family: Artists, photojournalists, wimpy sensitive guys named Wind, not to mention their numerous other relatives... They didn't show up often, but Jane and her brother Trent (who seemed to have run of the house for the most part) were often the only two rational Lanes at the scene when they did.
- Kim Possible: Shego's brothers have the ultimate embarrassing secret: She used to be good. However, their quirks (namely Hego's adherence to comic book style heroics and Mego's complex regarding his powers) along with her own troubles and impatience leads her to leave. She does still care about them, enough to where its implied she let Kim strip fof the rest of her brothers' powers so they can have them again (though she didn't like that being brought up.) Dr. Drakken has his own prodigal family, to a lesser extent: his mother and Motor Ed. (They're cousins. Seriously.)