Maybe your brother reminds Aidan of someone that he associates negatively with.
EG, if a dog was abused as a puppy by a man with a moustache, later in life when he sees men with moustaches he may become nervous and start growling. You'll often here about "racist dogs" that growl and bark and people of one particular race. It's not because everyone of that race is bad, but just that they associate a certain skin color with something that happened to them earlier in life. It takes time and a lot of difficult work for dogs to overcome these stereotypes. Different dogs have different triggers. Facial hair, baggy clothes, heavy coats, baseball hats, skin color, weight, odor, it could be anything. But you're right, it could be the person. Dogs can sense when people are uncomfortable around them, they can sense fear, they can tell when someone doesn't like them. I don't know if they can sense that "this is a bad person" but they can tell when someone is shifty. In addition Aidan knows you very well, and you mentioned you hadn't seen your brother in a year, so maybe he's sensing something in you that doesn't sit right. I'm not saying this is the case, but if you were expressing any internal resentment or disgust at your brothers sudden appearance then Aidan would have picked up on that and tried to play the part of your bodyguard. So maybe Aidan senses something isn't right with your brother, or maybe he senses some reluctance on your part, or it could just be that he associates your brother with someone else that he had a negative experience with in the past. It's hard to tell.