As far as I can tell there's nothing my brother does to provoke the dog - for the most part he was just moving from one spot to another, not teasing or anything. And yes I have witnessed it - in fact this morning when we came up from the basement I was watching EL D (I was first up the stairs) and he was just fine then all of a sudden as my brother hit the top step (doing nothing other than watching where he was stepping) EL D charged him.
Is it always related to changing rooms and crossing thresholds? If so, then you can start to train an incompatible behavior. For the guest bedroom, I would probably use a baby gate so he can't practice the behavior and your brother can enter and leave the room in peace. For other room changes, if El D is food motivated and has a good sit or down how about asking for a sit or down and then lots of treats while your brother is entering the room. Also having your brother give El D treats for things like sit, down, etc so he starts to see your brother as a good thing. If he is coming in the room he gets treats, if he is around he gets treats so that him being around is a signal that good things will happen.
And it sounds to me like for some reason maybe your brother is giving off some kinds of vibes that El D is picking up on…
An old school technique my grenpeppy taught me way back when is to wait for your dog to mouth your hand and then grab his lower jaw. If you use your thumb to apply a light pressure on his tounge he soon learns that biting people's hand is a very unpleasant thing. to speed it up try putting bitter apple spray on your hands
An old school technique my grenpeppy taught me way back when is to wait for your dog to mouth your hand and then grab his lower jaw. If you use your thumb to apply a light pressure on his tounge he soon learns that biting people's hand is a very unpleasant thing. to speed it up try putting bitter apple spray on your hands
What does that has to with a dog nipping at her brother when he enters a room? Obviously something is setting this dog off, this is not just play biting….
And this is a very old solution that many trainers now days would not use
An old school technique my grenpeppy taught me way back when is to wait for your dog to mouth your hand and then grab his lower jaw. If you use your thumb to apply a light pressure on his tounge he soon learns that biting people's hand is a very unpleasant thing. to speed it up try putting bitter apple spray on your hands
Maybe for play biting (not a technique I use)…but not for aggression...unless you want to MAKE a seriously aggressive dog, and get bitten for real.
It sounds to me like ElD just doesn't like your brother. It happens. One of my dogs just doesn't like my Mom. Nothing she did, nothing in the dog's history. El D is trying to keep him from entering his space.
So, the best thing to do would be change his association of dislike with your brother to like. And the best way to do that is for you to find some kind of food that the dog LOVES, and feed it to him open bar (continual) as soon as your brother makes an entrance. Have your brother quickly leave, and you 'close the bar'. Brother comes back in 'open bar.' You get the idea 
I wouldn't have your brother feed any treats yet. You can get into problems if the dog is conflicted by being drawn towards the object (by food) that they wish to increase the space between. Does that make sense? So you be the food giver. And it has to be something FABULOUS….liver, turkey, steak...something like that, so that El D starts to think 'I love it when that guy shows up, and I WANT him to show up so that I get the great food.
During non training times, tell your brother to completely ignore El D. No trying to sweet talk him, no eye contact, don't sit near him, don't approach him, nothing. You may need to keep the dog on a light line/leash to keep him from getting bolder and actually deciding to bite.
I hope some of this helps
Thanks Andrea, I'll keep these hints in mind. My brother left to head back to Chicago before the next snowstorm comes in so I can't implement them until the next time he comes to visit. I'm convinced it's just something about the one brother, EL D doesn't do this with my other relatives.
Wizard-I actually have the same issue with my dog C3PO. I haven't put my finger on it yet but it seems that when ANYBODY enters our living room or leaves our living room it sets him off….like where are you going??!!!
He always goes for the person's knee & he doesn't nip hard so I know he's using some kind of bite inhibition to send a message to the person.
Yet when the person is sitting/standing in one place he's fine & LOVES for the person to pet & play 
Yes that is similar. When EL D nipped my brother it was never really hard (okay so he'll have a bruise) and he backed off pretty quick once he let my brother have it. But it is unnerving because it is unpredictable.
