I haven't been on here in forever but am excited to see your post! When are you due? Congratulations! We liked "I'm a big brother". We also had another but I can't remember specifically the name. It was the same idea. Just anything to talk about what a cool job it is to be a big brother and how Mommy and Daddy love both of them very much.
I'm not parent yet, but I can recommend some based on being an early childhood teacher.
A Pocket Full of Kisses (sequel to The Kissing Hand, also great) by: Penn
Julius: Baby of the World by: Henkes