Wireless signals travel out in all directions from the WAP (Wireless Access Point) to any device that can receive the signal before it deteriorates and dies in the air. The only thing that's going to matter is how strong the signal is to each device. The weaker the signal, the more packets (data) that will drop and cause issues with net related issues, which can be quite annoying with games. So as long as they're both getting good to strong signals, there won't be an issue.
However bandwidth (Your 15.5 Mbits/sec) will be open for all to use that are connected to your router/wap. If your brothers laptop is downloading torrents/porn/whatever, then it's going to kill all the bandwidth to your PS3 during that time. More or less I'm saying that it's a shared speed across all devices on your network but in no way are they "split" unless you have some advance configuration options on your router/wap that allow you to set max speeds to devices.