Sibling Rivalry
As a companion to How To Talk So Kids Will Listen and Listen So Kids Will Talk, by the same authors is Siblings Without Rivalry. This book uses similar strategies, but more specifically applied to issues that arise when dealing with siblings.
For a 'fun' picture book that frankly addresses feelings of older siblings try Julius, Baby of the World by Kevin Henkes.
and for something a little more subtle, try A Place for Ben, by Jeanne Titherington: