I kind of don't know where to start.....
When she INAPPROPRIATELY 'I've her her vicious bark at strangers,' how are you correcting that.
Because if you are not.
If you, instead, are LOVING that your GSD is 'protecting' you.... then I don't think you are ever going to 'fix' the problem you think you have with family members.
Puppies, and you have a puppy. Have ZERO, I mean ZERO
skills and abilities to ACCURATELY know friend from foe. They just don't.
So if YOU have not been showing them that everyone they meet, EVERYONE, is just wonderful... but instead been allowing your UNPREPARED puppy 'decide' scary strangers from not... then you have given permission for your puppy to disregard YOU and it's all up to them. So why are they going to listen to you when you say your fiance is ok?
Either you are in charge and your puppy knows EVERYONE is wonderful cause you say it is... or you are NOT in charge and you've given it over to your puppy to decide.
Your pup is NEVER off leash in the house with your fiance? How many miles/hours of exercise is she able to get? If you have a high drive intelligent bouncing off the walls pup then that may be adding to what you see too.
How far have you been able to go on any of this training? --> Top Training Expectations for Puppies