Occasion
For the child who is about to stop being the youngest. These stories take the resentment seriously rather than insisting on delight, which is what makes them useful in the weeks either side of a birth.

Ages 4-7 · 5 minutes
The umbrella is small. The walk is long. The rain does not care.
Ages 7-11 · 10 minutes
Eleven minutes a day, and only two of you can go.
Ages 3-7 · 5 minutes
Everyone in the herd is enormous. Compsognathus is not.
Stories that assume excitement leave a jealous child alone with their feelings. Choose ones where the hero is honestly annoyed first.
Two or three weeks ahead is ideal. After the birth there is rarely a calm ten minutes for a new story.
Enter your older child's first name and Imagiverse writes them a story where the new baby arrives and they are still the hero.
Around four to six weeks before the due date for under-threes; earlier is mostly abstract to them. Stories are the easiest on-ramp.
Yes — a personalized story can name both children and give the older one the hero's role, which is usually the point.