Just some suggestions

* Big Sister hair clips - PM Me if you like i know a FB business that does these
* Big Sister bracelet or necklace
* Big Sister Photo Frame - she can pick her fave photo to put in it (fitting as she's your official photographer)
* You could buy the printer ink that you make your own deisgn, print it and iron onto a t-shirt... she can choose her own deisgn or slogan as her 'big sister' role.

Umm i'm sure i can think of more

FWIW when my mum was having my little brothers (i was 12ish) i was a little jealous, letting me pick some presents for bub and making sure we were the first ones to know about him made a big difference.. no-one else was allowed to see him til we did