J.,
My son was born just 5 weeks after my daughter's second birthday, so I feel you on this one! Here is what we did.
For her birthday, do something special with her, not necessarily a gift, but something with just mom and dad. This will be one of the last times she will get that.
We put together a very special hospital bag for her, too. We knew we would be spending a lot of time at the hospital when baby Shawn was born, so we made sure we had a couple gifts for my girl to open and play with while we were there. We also had a couple new outfits for her so she could wear new clothes home with her brother.
This really helped her with the transition and she loves being a big sister now. She has always been a wonderful little helper. She saw all my ultrasounds as I got them, so she felt like a big sister from early on.
I hope these ideas are helpful.