The Sweet Bee has read this book multiple times. It’s perfect for a two and half year old. It does a good job of explaining what a little baby is like (“Too little to walk. Too little to talk.” etc), and emphasizes the important job of a big sister.
Julius the Baby of the World by Kevin Henkes
Lily doesn’t see anything special about the new baby, and just wishes he would go away. But she eventually has a change of heart and becomes his fiercest protector.
A Baby Sister for Frances by Russell Hoban
With a new baby in the house, no one has time for Frances anymore. She decides to run away (to another part of the house) but comes back when she hears how much her parents miss her.
Check out this list for more books about siblings and new babies.