It sounds like that is attention seeking behaviour. Even the idea of regurgitating makes it seem like she sees you as a mate, and doesn't like the idea that your brother is taking attention away from her. The best bet would be to try and get her used to being around other people, and being able to socialize with them; even if that's something as small as just having your brother stand next to the cage and chat with her for a bit.

I have a blue and Gold Macaw that I've had for couple of years. She's very attached to me and whenever I let her out of the cage, she bobbles her head and sometimes even regurgitates. However, whenever my brother is vising me, she changes completely and can't stop biting me. My brother doesn't even take her out of the cage or anything, but whenever he is around, she's ready to bite me. As soon as he's out of the picture, she goes back to normal. What can cause this behavior?

Thanks