Kelly, thanks for the link. I had already read this article. It is more relevant to slightly older and more verbal children, though who are going to be even more aware of what's going on, maybe even consciously remember the experience for the rest of their lives.
She says that
I could wax on for ages about the social, emotional, cognitive and physical developmental benefits for children who witness the birth of their siblings, all of which are valid and factual points to make...
This is exactly the kind of info I am after.

Saša