I would just keep it to those people. ie: you, her friend, her, and the Husbands.
You don't know her family or what relationship she has with them.
I would not contact them via Facebook.
Ya never know, what a family's dynamics are. And it could be nice or not.
You don't know.
So I wouldn't even contact, her family. And you don't know who she likes or not in her family or who she has problems with or not etc. or clashes with who or not, etc.
And to ME, it seems like it would be a really nice intimate shower, for her, with just you all. AND the men can attend too. With my baby showers, all the Men attended too, and they really enjoyed it.
Remember, that the memories and quality of a baby shower, is NOT contingent upon the number of guests nor the size of the party.
And to me, if it is a small intimate baby shower, you can perhaps "spend" more on special decorations or gifts or mementos, for her, from ALL of you.
www.etsy.com is a great place to get nice "Mommy" jewelry. Maybe you can all get her a nice Mommy pendant/chain etc.
A nice photo album, etc.