Have you implemented any wireless security settings, such as encryption (WEP/WPA), hiding your SSID, MAC address filtering, etc.?
If your wireless router is running DHCP, configure it to only issue as many IP addresses as are needed by those in your household (I guess your PC and your mom's PCs). You definitely want to implement WPA encryption if you haven't already done so.
Try plugging into your router with a cable around the time the "wireless" gets slow. If you notice that your bandwidth speed is reduced even when connected via cable, then the problem would most likely be your general Internet connection, and not just the wireless.
I see that you mentioned that the speed goes from 54 to 11 Mbps. It could be that your wireless router is rated for 802.11g, while your mother's wireless router is only rated for 802.11b.
802.11b
Release Date Op. Frequency Data Rate (Typ) Data Rate (Max) Range (Indoor)
October 1999 2.4 GHz 4.5 Mbit/s 11 Mbit/s ~35 meters
802.11g
Release Date Op. Frequency Data Rate (Typ) Data Rate (Max) Range (Indoor)
June 2003 2.4 GHz 19 Mbit/s 54 Mbit/s ~35 meters