The GSD, Orion, has a history of abuse and neglect and is very skittish around people and loud noises. However when I walked in the room, he came up to me, sniffed, and then climbed on the couch, and put his head in my lap. He has since followed me around and been protective of me around the other dogs. He seems to have formed an instant attachment!
While it's true dogs do pick a special person, in this case, it's desperation and he was guarding
you, not protecting you.
If anyone takes/cares for him, they need to learn about the 2 week shut down and start it immediately.
Dogs like this one are in so much transition it's rather like a drowning person clinging to a life-ring.
They don't stop to ask what color the life-ring is, they grab and cling on.
We discourage that type bonding in our rescue because it is not a healthy one, and when you see a healthy one, you'll know the difference.
The "protection" at this stage very easily transfer to humans and within a week or two you'll have a mess on your hands.
This is a PDF file and a bit longer to load than an html page, but please try to get it loaded and print it off, and follow it, or have your sister do so.
Chances are, living in her home for 6 mos., he's going to be her (or her husband's) dog by the time it's done. And they'll see this "bonding" (guarding) as well, and if they have other dogs, it's going to be a mess.