Please help me understand my German Shepherds protective instincts
Not sure if this is the right area for this question, but i am going to ask it here since i could not find a place i thought more appropriate to ask, so here goes. My mom bought a German Shepherd about 2 years ago, and i have a question regarding her protective instincts. I am living with my mom while going to college and have helped raise Nymeria (the german shepherd in question) since she was first brought home. My brother Randy however, has only recently started staying with us for the past three months while he prepares to go back into the army, and her protective instincts are for him, and not me or my mother whom she sleeps on the bed with at night. We tried a test after his girlfriend playfully smacked him in the arm, and Nya started barking and ran at her. She was of course calmed down first, but then my brother Randy smacked me, and she jumped at me and started barking at me. Next, my mom, whom she is the most attached too, was smacked in the arm and she started barking and jumping at my mom! this is so wild to me. Randy is rough when he wrestles around and plays with her, is this protective instinct to Randy because she sees him as the Alpha? i always thought she would be most protective of my mother, but that is not the case, whoever randy touches, she barks and jumps at. What is the reason for this? can someone please help me to understand?
P.S- I know that this could also be an upsetting experience for her, so we have only just tried this experiment and do not plan to repeat it, but we are just curious as to why she is acting that way towards my brother, instead of my mother.