Some people, especially when they are younger, actually feed on dramatic, stormy, crazy relationships. If he is stressed out and upset about how she is acting, yet doesn't want to break it off, then he must be getting something
out of all this. Maybe on some level he enjoys all of the excitement and drama. I had an ex-boyfriend who's ex was just as nutty as a nut can be, sounding a lot like the girl you describe. My ex would tell me all about her, and I thought he was happier with me because I was calm and peaceful. Then reality was that he thought our relationship was dull because I wasn't regulary flying into rages, calling him at all hours, stabbing him etc.
For some people, any attention they get in a relationship is good, even negative attention.
You can't learn someone else's lessons for them. Either your brother will see the light one day, or else he just has a thing for bat-***** crazy women. He might have a lot of issues himself, if he wants to be with someone like this. If he's still in high school then your parents can forbid him from seeing her.
stand in the shower acting out her day that she just had - do to anger
Just out of curiosity, how do you know what this girl does in the shower? I talk out loud to myself in the shower, hope that's not too odd.