Technically, the Xbox can and will only connect to something with an Ethernet port.
if your brother has a device that provides a dial up connection on one end and an ethernet port on the other end, it could work.
If there is no such device, try to do it with the PC.
Look, if the PC has an Ethernet network card, and connect this port via an Ethernet cable with the Xbox 360.
On the PC establish a dial up connection to the internet.
Now use the Internet Connection Sharing wizard (search for ICS in the Windows help for it), to allow the PC to share the dial up connection with the Xbox.
This way, your brother can get downloads from the marketplace *very very* slowly.
However, online multiplayer will have *so* much lag, that it will nowhere near playable or even remotely enjoyable.
For the absolute minimum, you will need about 380kBit to make online play more or less possible.
A dial up modem will have 56kBit at MAX. So, voice chat, multiplayer in any game will not be enjoyable or even possible.
As long as you don't need real time response, your brother can send you a Live message in text or recorded voice form. But live chat will probably not work either.
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