What it looks like
It is rarely jealousy in the way people expect. More often it is a child who was the whole world for three or four years, and is now sharing it with somebody who cannot even talk yet — and who gets picked up every time they cry.
The behaviour that follows tends to be about attention rather than the baby: going back to baby talk, sudden clinginess, accidents from a child who was reliably dry, or a flatness that is easy to miss because it is quiet and convenient.
What tends to help
Naming the feeling for them, rather than correcting it. A four-year-old does not have the word for what they are feeling, and being told they love their brother really does not give them one.
A job only they can do. Not a chore — something with status. Choosing the baby's outfit, being the one who knows which song works, being first to spot a real smile.
Time that is not shared and is not earned. Twenty minutes that would have happened anyway, doing something ordinary, without the baby in the room.
Being talked about, not just talked to. A great deal of a new sibling's first months is a house full of adults discussing somebody else within earshot of a child who used to be the subject.
Where a book comes in
A story is one of the few ways a small child can look at a feeling from the outside. It happens to somebody in a book, which makes it safe to think about — and if the somebody in the book is them, by name, with their hair and their house and their actual little brother, it stops being a general story about siblings and becomes a story about them.
That is the whole of what we make. You describe the family; the story and every illustration are written and drawn for those people. Nothing is a template with a name dropped into it, which is why the book below is about Ava and Diego rather than about a child and a baby.
