E.R.
G.,
I don't know where you live, but I do know that they sell "I'm a big brother" shirts typically in hospital maternity ward gift shops. I know I saw them on the way out of Cedars-Sinai when I was bringing my newborn home.
Also, they may have something at the baby onesies cart at The Grove. The particular cart I'm talking about is infront of Barnes and Noble or Anthropologie. There's only one onesies cart, but they sell all sorts of funny baby shirts, onesies and other items.
Hope you find what you need!
All the best,
E.