First off, congratulations! When my second daughter was born, my oldest was almost 3 so she could understand a bit better. What we did before Allie was born, was tell Sophia there is her baby sister in my belly and when the baby is born she is going to need a lot of help dressing etc.. I bought her a sister doll, one she can play with when the baby is born. I got one that had diapers, could get bathed etc. that way when I was changing a diaper, she was too! After I had Allie, my sister brought Sophia to the hospital. I came out and said to Sophia, would you like to meet your sister? She came into the room and I put her on the bed. Sophia held her. Best moment of my life! When we got home from the hospital, I let Sophia help me as much as she could. She got diapers, wipes, and I let her pick her sisters clothes. Then when she was napping I spent time with Sophia. Now Allie is 2.5 and Sophia is 5 and they truly are best friends!

I am in the middle of dealing with this very issue. I have a 16 month old DD and now have a 2 week old DS. While I was pregnant, I would tell her there was a baby in my belly and she was going to be a big sister. I don't think she understood any of that. When my son was born she met him during daily visits to the hospital (I was in hospital for almost a week with him). She waved and said "Hi". It was cute, but that ended.
Now she wants to poke the baby while saying the word baby. She steals his blanket, paci, or anything that appears to be his. She is wanting to share her food with him by attempting to jam it in his mouth. She also has resorted to screaming loudly when the baby cries. She also wants to be sweet and give hugs and kisses, but puts too much body weight on him.
We limit the contact she has because she is so rough and I really fear she would hurt him. I put a baby gate up in his doorway to protect him and keep telling her to be gentle.