Get Your Own is probably the best idea then you can position the antenna any way you want (or even, preferably, stick a cable into your system). There's not a whole lot you can do if you're 'stealing' someone else's wifi.

Check your bro's wii, if it can connect right now (check the news or weather or shop channels), put your PS3 where his wii is and try again. If it can't, whoever you're piggybacking on probably put a password on it.