I believe that you want to build a WDS (Wireless Distribution System), connecting you Airstation as a wireless bridge with your brother's airlink?
This is sometimes difficult depeding on the routers. Some requiere to input MAC addresses and to change some wireless advanced values.
I've found a user's manual for a Buffalo G54 router, check it out, look for the WDS bridging section (page 67).
http://www.buffalotech.com/support/g...l-v1.7-web.pdf
Good luck.