I have a large rescue dog (American bulldog type) and she is usually lovely and friendly towards everyone, including other dogs.
However when introduced for the first time to my sister at our parents house a few years ago, my dog kept barking at her. When sister fed the dog some treats in an effort to make friends, my dog just quickly ate them before resuming the same barking.
Then when we were all sitting in the lounge together watching tv, my dog kept standing up kind of stiff and looking over at my sister who was on another sofa, and barking some more. My sister was uncomfortable with it so I put the dog on a lead next to me. I tried to shush her but she still barked, so I put her upstairs for a bit.
Later on that evening dog was downstairs again sitting at my feet, under the dining room table. My sister walked through the room and my dog got up and ran toward her. My sister ran away shouting some horrible stuff about my dog, and slammed the door behind her.
I think the shouting upset my dog so I comforted her a bit and she settled down peacefully again.
Since then my sister has refused to be around my dog, because she is scared of her. It has made short notice family meetups quite difficult as we have to coordinate visits ahead of time with a dog sitter. This barking stuff happened years ago and my dog has never reacted that way towards anyone else since. So I'm not sure what to do, if anything. Any advice?