She will adjust eventually. I would take time to talk to her about the baby before your LO arrives. Tell her how she can help you when the baby gets here. She can help bring the baby blankets and give the baby kisses, read the baby books etc. You can make a calendar counting down the days until the baby arrives. Make it two weeks after your due date and if the baby comes early you can tell her the baby just couldn't wait any longer to meet her big cousin. You could also get her a special cousin gift to give her from the baby after she gets here.
Just be sure never to leave her alone with the baby when your LO arrives. She's still too young to be left unsupervised.