I'm going to tell my brothers girlfriend to mind her own business
Really? How do you think she will react?
Her reaction is irrelevant.
I understand that you feel the need to tell her she needs to mind her own business but the way she reacts to it can affect your bother. There might be a nicer way you can approach this to get your point across.
I don't give a damn about her feelings.
I'm sure you would want your feelings taken into account if someone told you something like this, so you should try to treat her the way you would like to be treated. Being kind to others can go a long way, whether it be avoiding conflict or having the person be more agreeable. Try to think about that before you make your choice.