There's nothing wrong with a dog being more closely bonded to one person, but the possessiveness is something that definitely needs to be taken care of asap before it escalates. She is, like you said, possessive of you and thinks she is the boss and needs to take care of you, when it needs to be made clear it's the other way around.
This is something I've never dealt with personally, as Jackson has never growled, bit, lunged, or anything at anybody, even though he is very attached to me. My first suggestion... when this happens, for example when you say your fiance lays on the ground next to you guys and she growls or nips, you immediately get up and walk away. If she is in your lap and growls at someone coming near, she immediately gets put on the floor. Never reward the behavior. Alot of times people don't even realize it but when their dog is in this state of mind, many people pet or rub the dog and say 'it's ok' or 'be nice' etc when in reality... the dog thinks they are doing good since they are getting 'rewarded' in their mind for this behavior. If she wants picked up while doing this behavior, ignore her.
Implement NILIF (Nothing in Life is Free) program. You can google it.
Before eating, she must work for it. Make her sit and be in a calm state of mind before eating. Or exercise first, then eat. She does not get a bully stick until YOU tell her, and she also sits before getting this, etc. But that program works for quite a few people.
Nothing in Life is Free