A great idea for a big sibling present is the book "Alligator Baby" by Robert Munsch. My oldest got it when my second was born, and he loved to read it. It's about a mom and dad who leave to go to the hospital to have their baby, but they come home with various animals, instead of the baby. Eventually the big sister leaves and finds her new baby brother and brings him home. It's a cute and silly story.
The shirt ideas sound great to me, as well!