Cindy's Answer
It's not possible for me to know why he's like this but it's not an uncommon issue. If dogs aren't given structure, training and rules they will often become overly territorial and that means aggression for some individuals. Boston Terriers are tough little dogs with a lot of drive and energy. Sounds more like he has a history of overly excited behavior when greeting people and excitement can very quickly turn into overarousal which can lead to an aggressive incident.
Dogs like this need training and supervision and since he's been practicing it for a while it can be much more difficult to work on than if he had been given the right type of training from puppyhood.
We have a lot of resources for managing and training dogs like this, if your sister wants to change this dog's behavior it will require daily training and management and taking away some of his freedoms so that he can stop practicing the wrong thing.
Sidenote, rabies in domestic dogs in this country is extremely rare.
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