Chances are your girlfriend will chose her family anyway, this is a textbook example of domestic abuse, it has taken years for her brother to degrade her to the level where she puts up with that, I'm familiar enough with japanese women to know most are quite verbal and strong-willed, faced with a man or otherwise, when they want to be.
I bang heads with my girlfriend all the time, two strong willed people, and by god I love it.
I think your best bet is to get your girlfriend away from her brother, and maybe family, all you can do is hope you can get through to her and convince her to stand up for herself, even if that means distancing herself from him, he is clearly not all there, possibly a bit unstable, way too controlling beyond all reason.
The problem is you havnt known her that long, and you should only considor this if you have any intention of you and her being a long term, hoping for life kinda deal.
Thats not really a decision you can make out of the blue though.
Perhaps you could get her friends to help you convince her to go her own way, or to get her parents to take some action against him, but he is their only son, in Japan you dont take an only son for granted.
Tough situation I dont think i can really offer any serious advice, and nothing beyond what I've already given, hopefully you'll figure something out.
Best of luck.