Little kids handle big changes better when they know what's coming. A story they can hear again and again, on their own schedule, does a lot of that preparing for you. And when the main character has their name and their face, they don't have to imagine themselves into it.
Generic big-sibling books are full of good advice about a baby who isn't theirs. Yours can say the true, specific things: the baby will sleep in the room with the yellow curtains, Grandma is coming to stay, and they still get to pick the bedtime book.
It also gives you words. Reading a story out loud is often easier than explaining from scratch, especially for a two- or three-year-old.