She is a doll!!
Sometimes the runt does act like this as they age- . The reason is that the little girl had to fight to survive. But do not worry- it planes out. That is why I keep my puppies a little longer- gives the bullies as younger pups, to get " one upped" by the smaller ones as they mature. It planes the field out so to speak.
She is precious! I take it- she is the one you are keeping?