My brother's family has a 1-yr-old English Sheppard. She's a lovely dog most of the time. We spent a week together at a cottage recently and all was fine...until my other brother arrived.

We all arrived on Sunday, except for the other brother. The dog has only met my husband and I a couple of times. At no time did the dog have any problems with anyone.

For ease, let's call the brother who owns the dog B1 and the other brother B2.

On Tuesday night B2 arrived. He arrived late at night, in the dark and tried to just walk into the cottage. I guess he scared the dog because she freaked out at him. She started barking and growling and tore a hole in his shorts before B1 was able to get hold of her collar and leash her. B2 sat in a chair near the door and tried to be as unthreatening as he could. He didn't address the dog after that, except to give her some treats to try and gain her trust which she hesitantly accepted. But she continued to bark and growl at him anytime he moved.

The next morning she seemed to be over it. We kept her leash on but let her wander freely while someone held the leash to pull her back if needed. She walked up to B2 and sniffed at him and she seemed okay with him. She was relaxed around him and everything was good. But then B2 accidently knocked a water bottle over on a glass coffee table and she started barking and growling at him again. It was such a sudden change that it made us all nervous to have her around him at all.

We thought the problem was just with him. But then she started barking and growling at the couple we rent the cottage from. She had been perfectly fine around them up until B2 arrived and scared her.

My sister-in-law went out and bought a muzzle for the dog. For the rest of the week, anytime the dog started barking threateningly at someone, she would quickly get muzzled. It was partly to protect whoever she was barking at, but also to show the dog that the behaviour is not acceptable. The muzzle would stay on until the dog calmed down and then it came off. After a couple of days she seemed to be getting the hint. B2 had gone home by then so we don't know whether the calming down can be attributed to the muzzle or just to the fact that the threat was gone.

My questions are this...
Was muzzling the dog the right thing to do?
Is there a way to train the dog that this aggressive behaviour is not acceptable?
We assume that her aggressiveness is because B2 frightened her and she was being protective of the rest of us. But could it have been something else about him?
Why would she start showing aggression to the couple that we rent the cottage from when she was perfectly fine with them the three days prior to that?

Apparently, this happened once before with someone that B1 and his family went camping with. The dog just took a dislike to one of the other people on the camping trip and they had to keep her far away from him.

Dog ownership is new to our family. We've generally been cat people up until now. The dog is smart and learns quickly but we need to know how and what to teach her in order for her to learn.

Any help or suggestions would be appreciated. Thanks.