Waterdreamer did a review a while back about the Worrywoo monsters (?) a while back a while back, which is a line that teaches about emotions in general. Butt I bet there are some great books out there about adjusting to becoming a new older sibling, maybe someone else can chime in.
Yes I did. It is a very cute series.
http://domanandaba.blogspot.com/2010/11/monsters-in-my-head-worrywoo.html