I would say no to the shower. My oldest was 20 months when I had my twins and we did not have a "shower". I felt it was a bit much since I just had my shower for my first DD less then two years prior.
My close girlfriends had a party for me just before the birth but it was about me (they had someone come to the house for mani/pedis and got a gift card for a cleaning service).
I know after we got home from the hospital everyone brought tons of clothing and gifts when they came to visit.
If you would like to do something fun do a "Big Brother Party" for your son....maybe people could bring him books or things to get him ready for his new job as a big brother. This way the birth of your new DD is celebrated along with making the day special for your little guy.
I went to a "Big Sister" party and thought the idea was very cute and a special way to make her feel a part of the whole new baby thing....
Good Luck & Congrats on your new DD
Jen