Do you have any links? I remember reading lots about at what ages (ballpark) toddlers were capable of grasping certian concepts, but I've forgotten it all. I know it's kind of up to the individual child, but it will be easier to explain to my almost-five year old why new baby doesn't/can't do certian things if I can get refreshed on developmental stages.
Need some age-appropriate behavior resources
Here are a couple of articles that touch on this subject:
And, I'm wondering if your almost five-year-old might like watching the film The Connected Baby as it might give him or her a good sense of baby development. Here's my blog about it. We're not streaming it currently but you can find out more at their site, linked above.
I found it helps including siblings in the pregnancy and birth plans. Here are a couple of reprints in our Mothering Shop on the topic:
Hope this helps.