A child really wants a younger sister or brother, often to their parents' chagrin.
often quite common in fiction, leading to a lot of only children running around. But some fictional only kids (especially younger ones) are not happy with the situation. Maybe they're lonely because they have no one to play with. Maybe they're frustrated because they have no one to push around. Maybe they just see a lot of other kids with siblings in real life and are just jealous. Whatever way, they know what they really want: A younger sibling. And they aren't afraid to ask their parents for one. Their parents are generally less thrilled about this, because even if they also want another child, they're not keen on telling their kid where said baby is going to come from. Frequently, kids will express a desire for a sibling the same gender as them: Girls generally want little sisters, and boys typically want little brothers. But the opposite situation isn't unknown: Sometimes boys want little sisters in an attempt to purposefully invoke Big Brother Instinct. Similarly, sometimes tomboyish girls will want brothers... and not-so-tomboyish ones want living dolls. In works with a fantastic bent, this may be a cue for a younger sibling to suddenly come into the picture through supernatural or superscience-y means. In less-fantastic works, it's often up in the air whether the kid gets their sibling or not. Nor is there any guarantee that, should they get their wish, they and their new sibling will get along the way the older one dreamed. Compare I Want Grandkids (for when it's the parents' parents doing the pushing). This is also Truth In Television, which is probably why it shows up often enough.
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