u/Alternative-Cable-72

So I have two American Bulldogs, a female and a male. They turned one in February, and the female is spayed. The male is not fixed yet, but he will be soon. Of the two, the female is the more aggressive one, but neither of them have ever bit anyone. Wha she does, is she will attack her brother when a stranger approaches, only if they’re with my mom. She never does this around family only around strangers. Is that her way of trying to protect my mom, but if so, why does she go for her brother instead of the person? Is it a form of resource guarding in a way? Or trying to assert her dominance over her brother? I’ve told my mother time and time again that she needs to show these dogs that she is dominant. She sleeps on the couch with them every night because she watches TV and then just falls asleep next to them. She lets them do whatever they want and she lets them control her and I’ve tried telling her that she cannot do that because they will not respect her as a person if she does that. She just has a hard time asserting her dominance over these dogs. What can I do to stop this behavior? I should note that she only ever does this outside when they’re on walks and when a stranger approaches. Any other time she’s with her brother she absolutely loves him.