Sylvia is so sweet, especially with the nickname Sylvie. Great choice!
I don't think Sarah goes with it at all, which stinks because that's your husband's favorite name. Maybe you guys can save it in case you have another daughter, Sarah and Sylivia sound great as a sibset!
For a middle name, Sylvia Josephine was what I frist thought of. Also, Sylvia Elizabeth is pretty, though a bit more common.
Sylvia Zarita (Zarita is the Spanish pet form of Sarah)
Sylvia Sadie (the English pet form of Sarah)
Good luck to you, and your family has great names, congrats :)