Because Same Surname Means Related, an entire fictional family will always share the same surname, even grandparents, aunts, uncles, cousins, etc. Even in cases where it wouldn't make sense, most often with relatives through their mother's side of the family.
If writers want to attempt to be realistic by avoiding this discrepancy, they'll make it a Single Gender Family
with all blood relatives sans the matriarch being male because Lineage Comes from the Father. Any aunts are either an aunt-by-marriage or a Maiden Aunt. First cousins almost always share a surname, indicating that they would be the father's brother's kidsnote . Now extended relatives beyond that, such as great-grandparents, great aunts/uncles, cousins once removed, second/third/fourth cousins, etc. will almost always still share the same surname, indicating that they're all related through the male bloodline!note
Of course, that's not to say maternal relatives don't exist in fiction. As far as surnames go for them, it could come in multiple flavors: the character's father is out of the picture and thus shares their mother's maiden name; the character's father Took the Wife's Name (which is often implied); or their mother's maiden name is unknown thus the maternal relatives are only referred to by their first names or nicknames. If it's the third option, this allows for freedom for the writers to make relatives of any sex without worrying about discrepancies. In the cases that the mother's surname is known, she almost always will still have brothers with kids rather than sisters, as well as great-grandparents or great-aunts/uncles (or other extended relatives) being through her father's side. Both of these options make it easier for the writers to only have to keep track of one or two surnames for the sake of convenience and simplicity.
And in some cases, although rare, a woman's surname may be the same as her maiden name due to the fact that Same Surname Means Related isn't always true and there are plenty of people who share a surname that aren't related. Thus, she may have happened to marry a man with the same surname.
Note that this is not the same as Same Surname Means Related, as that trope is about the fact that characters will only share a surname if they are related, while this one is the other way around in that characters will only be related if they share a surname.
Often paired with Single Line of Descent. May be justified in a Tangled Family Tree, especially if it involves incest. The Three Successful Generations trio often all share the same surname (typically being son, father, and grandfather). Compare Lineage Comes from the Father and Ridiculously Long-lived Family Name. See also Nom de Mom, Took the Wife's Name, and The Maiden Name Debate. Species Surname might be a reason for subversions and other plays.
Examples:
- Yosho from Tenchi Muyo! has the same last name as Tenchi and his father despite beingng Tenchi's maternal grandfather. It turns out he's an ancestor of Tenchi's father and they're descended from Long-Lived aliens that practice incest.
- Marcel Dalton, the titular character of a Lucky Luke album, is an uncle of the Dalton brothers. He's a banker from Switzerland and the family's White Sheep. He's called Dalton even though he's Ma Daltons brother.
- Encanto: La Familia Madrigal. The grandchildren are also called Madrigal, even though it's their mothers that are called " Madrigal". Justified in that Abuela Madrigal and her children are the focus of the magic that keeps the protection of the Encanto going. So adopting the name of the family shows the continuity of the family and the miracle.
- Stuart Little: Implied. When the Littles' extended family shows up for a welcome party for Stuart, they all use the same greeting ("Little high, Little low!" — "Little hey, Little ho!"), indicating that they are all Littles, even though realistically some of those family members ought to be from Mrs. Little's side. This could be justified by the idea that being a Little is defined by something other than strictly by blood or legalities, since Mr. and Mrs. Little tell the newly-adopted Stuart that, "They say every Little in the world can find the Little house, even if they've never been here before. It's just something inside of them".
- Discworld: Nanny Ogg never bothered to take any of her husbands' last names, and all her children bear the Ogg surname.
- Princesses of the Pizza Parlor: The Player Party meets two Tatannus women, but the stronger sign of them being related is that they have Family Theme Naming and that they're both royalty from the same nation.
- The Word and the Void: Lampshaded. Nest Freemark’s magic sensitivity has been passed down through the maternal side of her family for generations. Although this means no one further back than her grandmother has the Freemark surname (her mother did not marry), she still refers to this ancestral line as, “The Freemark Women” as a matter of simplicity.
- The Addams Family (1964): It's established in the flashback episode showing how Gomez and Morticia got together that Uncle Fester is Morticia's maternal uncle. Despite this, he is referred to as "Fester Addams" several times throughout the series (including by himself on occasion), rather than sharing Morticia's maiden name Frump. Most likely to avoid this discrepancy, all other adaptations (except the 1977 Reunion Show) would shift to Fester being Gomez's older brother. At that point, the biggest discrepancy was Fester not being Hispanic, but even this was addressed in Wednesday where he is played by the half-Venezuelan Fred Armisen.
- Wednesday introduces a different character: Goody Addams who is almost identical to Wednesday and like other female relatives, she is a psychic but shares the last name of her patrilineal line. The confusion continues in the second season, where her grandmother Hester stresses that Wednesday might be legally an Addams but is a Frump within but Mrs. Weems and Morticia imply that these are also "Addams".note The trope does not apply to Weems as she is a distant relative of Wednesday but that was a later addition and it is unclear if she was even aware of this fact at first.
- The Dukes of Hazzard: This trope has caused quite some headaches for fans of the show who tried to figure out the Dukes family tree. There's Jess Duke, who has four nephews and a niece (Bo, Luke, Daisy, Coy, and Vance), who are all cousins of each other, and all share the last name Duke. For that to be possible, Jess must have five brothers, but none of them are ever seen or mentioned.
- It's shown early on in The Fresh Prince of Bel-Air that ''Smith" is the maiden name of Will's mother and aunts, implying he has a Nom de Mom due to his father not being around. But when Will's dad eventually does show up, he's called Lou Smith.
- Gilmore Girls: All of the relatives discussed in relation to Lorelai and Rory, and both girls themselves, are Gilmores—with the exception of Rory's father, Christopher (Hayden). This is Played for Laughs when Lorelai comes across some family lore and realizes that her paternal grandmother's maiden name was also Gilmore; Emily confirms Trix and her husband were second cousins. Averted with Luke (Danes) and his nephew Jess (Mariano), the son of Luke's sister and her husband in what's implied to be an extremely short-lived marriage.
- Averted in House of the Dragon. Laena and Laenor Velaryon are Targaryens through their mother Rhaenys and are treated accordingly, namely their ability to command dragons.
- Star Trek:
- Star Trek: The Next Generation: In "Sub Rosa," the ghost/alien has attached himself to generations of "Howard women." However, it's implied that this is a biological relationship: mother to daughter (or grandmother to granddaughter, in Dr. Crusher's case), since the women are said to commonly have green eyes as a family trait. But following traditional rules (as we understand this goes back to the 17th century), the only way there would be generations after generations of women with the last name Howard would be if he attached himself to the wives of Howard sons — i.e. descending from mother-in-law to daughter-in-law. This should have been clear because he descends to Dr. Crusher (née Howard), who couldn't possibly have been the first woman in her family to take her husband's name.
- Star Trek: Deep Space Nine: Averted with joined Trill who change their surnames to that of their symbionts upon being joined with a symbiont.
- Świat według Kiepskich: Babka/Rozalia, the mother of Ferdynand Kiepski's wife, inexplicably shares his surname — in most episodes, at least.
- Chzo Mythos: In the series, all ancestors and descendants of important characters share a surname with them. One of these surnames, Somerset, even spans as early as the 16th century to as late as the 24th.
- Fatal Frame: Kunihiko Asou is a Japanese folklorist and occultist from the late 19th century. Throughout the series, starting with II, characters are introduced who are related to him:
- Fatal Frame II: Mio and Mayu Amakura are daughters of Shizu Amakura and Misao Asou (who took the Amakura family name).
- Fatal Frame III: Yuu Asou is Rei Kurosawa's dead fiancé, who, like his ancestor, is also a folklore researcher.
- Fatal Frame: Mask of the Lunar Eclipse: Misaki Asou is a female descendant of Kunihiko and lived on Rougetsu Island her whole life, even falling ill to a disease that only occurs on the island. Text documents reveal that Kunihiko travelled to the island in the early 20th century, but he did not die there.
- Fatal Frame: Maiden of Black Water: Averted with Ren Hojo, who is a male descendant of Asou, but does not share his name.
- The Sims 3: Most families avert this and have family members who have a different family name.note The Wolffs from Moonlight Falls are werewolves. Then there is a different Wolff family residing in Sunset Valley and Sugar Maple Coast who are explicitly related to them but are just ordinary humans breaking Werewolf Theme Naming.
- Cirque Royale: Averted with Diana Cashworthy and her husband Waldo Ashew. They each opted to keep their own last names (as Diana is part of the wealthy Cashworthy family, and her father Aries actually took his wife Gemma's last name). When they had kids, they blended their last names for them, meaning their two daughters are Griffin and Chimi (Chimera) Cashew.
- Big City Greens: Even though Bill and Nancy are divorced, Nancy kept the name Green rather than going back to her maiden name Mulligan. Maternal grandfather Nick is the only member of the family to not share the name.
- The Fairly OddParents!: Mr. Crocker has a nephew named Kevin Crocker, who is his half sister's son, even though he would realistically have a different name.
- The Great North: The show stars the Tobin family, which consists of single father Beef Tobin, his four children, and daughter-in-law Honeybee (who, ironically, kept her maiden name). Extended family members have been shown and mentioned throughout the series, including Beef's brother Brian, niece Becca, aunt Dirtrude, cousin Danica, great-grandfather Rutger, and various relatives of an unmentioned relation. However, they all share the Tobin name, indicating a Single Line of Descent.
- Gremlins: Secrets of the Mogwai: Sam Wing's maternal grandfather somehow still has the last name of Wing, just like his son-in-law and grandson.
- Phineas and Ferb:
- Phineas and Candace's maternal grandparents are named Clyde and Betty Jo Flynn despite being their mother Linda's parents and not their disappeared biological dad which hints their biological dad changed his name to Flynn or after Linda was either divorced or widowed, she changed her name back to Flynn and then Flynn-Fletcher via marrying Lawrence Fletcher.
- Despite Vanessa Doofenshmirtz's parents Heinz and Charlene being divorced, Charlene chose to keep her married name.
- South Park:
- In the Season 2 episode "Merry Christmas Charlie Manson", Cartman's great-grandmother Florence shares the Cartman surname with her daughter Mabel (Eric Cartman's grandmother), despite Mabel being married to Eric's grandfather Harold.
- Though only shown on her gravestone, Kyle Broflovski's deceased maternal grandmother Cleo Broflovski, who is explicitly stated to be his mother Sheila's mother, inexplicably shares a surname with her married daughter.