Exaggerated: This lack of siblings extends to the higher generations; there are very few uncles or aunts, resulting in even fewer cousins or other extended family situations.
About half to two-thirds of the characters are only children.
Only children represent a realistic minority, but most characters have only one plot-relevant sibling or half-sibling. Ida is the sole exception, with two older sisters who are rarely shown. Everyone says that Ida has a large family, implying that she is the only person they know with more than one sibling.
The main characters are only children, but most minor characters are not.
Sol is an only child (like many others) since sadly it is set in a time/world where the maternal death rate is so high that a women's first pregnancy is almost always a death sentence.
Sol and his friends are part of a fantasy species where women only have one egg, and are deemed infertile after their first child.
If the characters are orphans as well as only children, it could be a post-apocalyptic setting where the vast majority of the global population has been wiped out. If you're lucky enough to have survived, it's very unlikely that anyone else in your family would have too. And because of government brainwashing, no one has any memories of their deceased relatives.
Most of Sol's friends once had siblings, but when the Population Control law was passed, each couple was only allowed to keep one child.
The characters have no known relativesother than siblings. This is true of all characters, regardless of age.
Many of the characters' parents conceive after it is established that they only have one child each.
None of the main characters' siblings are mentioned at first, but they start appearing in later episodes. If they are older than the main characters or if the parents are divorced, it is likely that the siblings in question were unseen because they live in another city.
At first it looks like everyone is an only child, but then in one episode they talk about their families, and several characters are mentioned as having siblings. They're just unseen.
At first it looks like a one-child policy has been in effect for decades, but then it is revealed that Alice has an uncle.
The pregnancies are forcibly aborted due to Tropistan's one-child policy.
The parents break the law and have more children in secret.
To avoid an aging population, the government of Tropistan repeals the Population Control law in favor of a legal age limit; from then on, most people are only allowed to live for 60 years. For the elite, it's 70.
Discussed: "What? Why should you just assume I was an only child? Even today, you are statistically more likely to have at least one sibling than not to have any. So there."
Conversed: "Isn't it weird how almost everyone is an only child in these books?" "I know, right? There's no one my family can relate to."
Implied: All the main characters' parents (and most characters' grandparents) are shown, but never any siblings, and very few uncles or aunts.
The lower reproduction rate means that parents have one shot at passing on their genes; one tragic death at the wrong time can completely wipe out that branch of the family line. While a prophecy states that a descendant of one of the former heroes will become The Chosen One, the lines are inadvertently snuffed out before the prophecy can come to pass. Hello, End of the World as We Know It.
Ichigo is initially forbidden from accompanying Sol because she's her family's only child. Prophecy requiring her presence be darned; they're not going to lose their baby!
According to prophecy, only the descendants of the last band of heroes can defeat the emerging evil once and for all. Unfortunately, there aren't too many descendants left; the current generation is made of only children, which The Empire is hunting down one by one.
Prophecies can't be broken that easily: one of the original heroes has a single descendant who has secretly survived by pure chance. They just have to find him.
Sol reassures Ichigo's parents that she's in good hands, and they reluctantly change their minds.
Plotted A Good Waste: The lack of siblings is not just incidental on the part of the writers; it turns out to be an important plot point, and an early sign that this world is not what it appears to be at first glance.
Played For Laughs: As a Running Gag, Alice asks her parents why she's an only child. They always respond with some variation of "You're more than enough for us, sweetie." (Alice never seems to pick up on the implication that she is too misbehaved for her folks to risk bringing another, similarly naughty kid into their world.)
Played For Drama:
The lack of siblings implies that the human race is in danger of extinction.