Here are the basics - you need a cake, light refreshments (can be appetizers or just nuts and mints to go with the cake...depending on the time), decorations (boy colors obviously), a theme (ties everything together), games, present opening time and that's about it!  Also, you could give a speech or have someone else close to her give a speech. 

Another nice touch is to have people write a piece of advice about motherhood on a piece of scrapbook paper and then compile all of them into a scrapbook for her.

or a diaper cake. :)

they're super cute

It totally depends on your sister's personality.  You have food (could be just appetizers etc), you have cake, and silly games.  I'd have it sooner rather than later.  It's traditional to wait until later to have the shower in my family, I am not sure why.  I was like 8 months or so and I just wanted to leave after an hour.  I really wanted to enjoy myself b/c I could tell everyone worked so hard at getting things together for me.

At my baby shower we had a game called "break the ice" where we'd frozen those little pseudo bottles and binkies into ice cubes, and then had them in the glass of water.  Whoever's melted first got a prize. 

Then we had different candy bars melted into diapers and people got to smell and taste them if they wanted to see if they could guess which diapers had which candy bars, *the diapers were numbered* (No I did NOT taste them, lol) and whoever guessed the most got a prize.  *Tip: Don't microwave the diapers with the candy bars... I'd heat up the candy and put it on a fresh diaper, lol*

Take lots of pictures!! She'll cherish them!