T.B.
When I was pregnant with baby #3, I had my two children make a "big sister" shirt. I bought t-shirts from Michael's and bought stamps of various "baby" themes (bottle, bear, little baby shoes, etc.). I let my girls choose the color paints they wanted and as far as "bling" you can buy fabric glue and glue on some shiny things like rhinestones or whatever you want. Just put a piece of cardboard between the front and back of each shirt (on the inside of the shirts) so the paint doesn't bleed to the back side. I bought special fabric paint with a fine tip and I helped them stencil in the words, "Big Sister." It was sooooo much fun and they enjoyed making them!
If you want them pre-made, try a "google" search for "Big sister shirts." I hope this helps!