Outnumbered Sibling

This trope is what results when a character has a particularly large number of siblings, and they're the only boy/girl. This can often result in the character hoping and praying for a sibling of their gender, Parental Favoritism in either the character's or all the other's favor, and the character feeling either superior or inferior to the rest. More often then not, said character is either the youngest or the oldest of them all.

Subtrope of Massive-Numbered Siblings. See also The Smurfette Principle and I Have Brothers

Highly expect Annoying Younger Sibling and Big Brother Bully to come into place.

Examples of Outnumbered Sibling include:

Anime and Manga

Sister Princess. An orphan named Wataru Minakami discovers that he has 13 sisters, one of whom is The Mole, and not actually one of his sisters.
Quatre Raberba Winner of Gundam Wing is the youngest child of his family and has twenty-nine sisters. The catch is that all of his sisters are test-tubes babies due to difficulties with childbirth in space among early space colonists; Quatre was the only child born naturally, although his mother died during childbirth and the fact that he wasn't artificially reproduced was kept secret from him. However, Quatre is shown to dearly love his sisters and it is the near-death of his favorite older sister Iria (along with the death of his father) which drives poor Quatre over the edge.

Comic Books

Hope O'Dare from Starman has four brothers. She and her brothers are all cops and it's mentioned that she is the one everyone is frightened of.

Film - Animated

Thumper from the film Bambi has five sisters, which is reduced to four in the midquel.
Played With in Brave: Princess Merida is the oldest child and the only daughter of the royal family, with three younger triplet brothers. However, the main conflict of the film seems to be that Merida herself is a Tomboy being forced into a ladylike role by her mother.
At the end of Lady and the Tramp the title dog couple become the parents of three daughters (all Cocker Spaniels like their mother), and a fourth male puppy named Scamp (who is an indeterminate breed like his father).
- The exact same is true with Doppler and Amelia's children from Treasure Planet.

Film - Live-Action

Roberta from Now and Then had three older brothers and no sisters. The brothers are only briefly seen but it explains why she's such a tomboy.
An Our Gang short illustrates this trope with a hint of Values Dissonance. Mickey is picked on by his two sisters and hopes that the baby his mother is pregnant with is a boy. Somebody tells him that every fourth child in the world is born Chinese. He's upset by the idea of having a Chinese younger but eventually comes to accept it. Unfortunately from his point of view, his mother gives birth to another girl.
Adam Sandler's character in Punch-Drunk Love.

Literature

Ginny Weasly of the Harry Potter series is the only girl in a family of seven children.
Mark in Edward Eager's Half-Magic and Magic by the Lake. He is the only boy in a family of four children and his frustration with this is a occasionally a plot point.
In the Vorkosigan Saga, Ekaterin has several older brothers and no sisters, and this is mentioned as being common among Barrayarans of her generation as the result of newly-obtainable sex-selection technology. In contrast to all the tomboyish examples of this trope, she specifically mentions this as being part of the reason why she's a Shrinking Violet.
In The Bible, Jacob has twelve sons - and one daughter, Dinah.
- Though in some Jewish sources each of the sons, and Dinah, came with a twin sister.
Horton Hears a Who!: The mayor of Whoville has 96 Daughters and one son, Jojo.
Flopsy, Mopsy, Cottontail, and Peter Rabbit.
In the Malloreon, Belgarion and his wife are long-lived, and it has been prophesied that he would only have one son.
Jenna Heap from Septimus Heap is the only girl in a family with seven boys, and the youngest child as well.
Posy from The Hunger Games has three older brothers.

Live-Action TV

Tends to be mentioned as the background of tough female cops, such as Jane Penhaligan from Cracker and Kate Todd from NCIS. The idea being that their reason for seeking to compete in an all-male world is from attitudes gained in their childhood.
Another "tough female cop" example is Jo Lupo from Eureka.
In the Dollhouse episode "The Target", Echo's persona is implied to have this in her fake backstory; when Boyd asks her if she knows how to use a gun, she replies with, "Four brothers, none of them Democrats."
Theo from The Cosby Show was the only boy in a family of five kids but I haven't seen enough of the show to know if he angsted over it.
This is the basic premise behind the shortlived sitcom Odd Man Out: a teenage boy has three older sisters and no brothers.
Daphne from Frasier has... what is it? Six brothers? She talks about them a lot.

Video Games

Wendy O. Koopa is the only girl of the Koopalings in the Super Mario Bros. games.

Web Comics

Surama from Wurr has four brothers and no sisters, and three of them are older than her.
Summer Winters from Moon Over June traces her misandry to growing up with three older brothers and three younger brothers.

Western Animation

It's not "massive" but on The Simpsons Bart occasionally has pangs of this, as the a boy with two sisters and no brothers. One time he went to an orphanage and got himself an orphan to be a brother.
In The Smurfs there is Papa who has 100 sons one daughter, Smurfette. Later, the Smurfs create another sister called Sassette.
Lor from The Weekenders has so many brothers that she loses count of how many there actually are.
This is the central premise of The Loud House. Out of his parent's eleven kids, all whose names start with L, Lincoln is the only male.