Well seeing as your Mum and Sister are plugged in with a cord it could possibly be a few things.
The Wireless antenna on the router could be faulty.
The Wireless card on your laptop could be faulty.
Or very unlikely you could have a virus that could be doing this.
Find of wireless card name:
1) Select Start on your computer
2) Type “Device Manager” in the “search” field
3) In the following list, select the “+” button next to “Network Adapters”
4) You'll see your wireless card details, for example: “Intel Wireless WiFi Link 4965AGN”. If you can give us the name in the next reply