Have a look on here at cross compiling. It's says that it may be possible on Windows, but I'm not sure if it would work.
You may have to ask your brother to build it on the Mac or you could run a VM on your Windows computer and build it there. However, to do the latter, you need to have a license to run Mac OS. You get one of those by buying a Mac, so if you have a licence, won't need a VM ;)