I'm going to point out the warning signs that my brothers girlfriend is putting out
You should make sure first. Are you honestly sure about these warning signs?
I know them very well because I've been with her since I was 6 years old
It is good to try and warn people about others red flags. You might just need to let him fail so he can learn on his own if he won't accept your advice.
I should talk to my brother so he makes a wise choice.
You're right, you should speak with him just to let him know about the red flags. Although you should be okay with the fact that he may not listen to your opinion, but it is good that you care.