We bought our then 29 month old DD a necklace and ring...(costume jewellery). To be honest, i really dont think the new baby (her sister) needed to have bought her anything.
She was smitten with her new sister, excited to have a cuddle and a kiss. I just made sure i did the same to her.
I wont be buying anything for the girls when DD#3 arrives.....me and DH will make them feel special, i dont think a 'gift' will.....good luck.