Congratulations on your newborn baby!
Actually I'm thinking of this too...I'm getting married in a year or 2, my yorkie is now very agrressive (like yours) when my little niece and nephew came over to my house. But I've found out, eventually they will be fine, calmer once they know this is going to be permanent.
Mybe you can let your yorkie smell your newborn's clothes, make her recognize the scent and familiar with it.