Spyware: Even single router of same type will most likely behave exactly the same. Without doing a wireless survey one has no idea what other transmissions exist and on what channel.
P.S. Your sister can use inSSIDer to do a wireless survey.
http://www.inssider.com/downloads/
Unfortunately without a spectrum analyser this only shows Wi-Fi signals. It won't show analogue or HDMI video senders and non-Wi-Fi devices etc...
I wonder if it's a router that has the RF power output only at 20-50% set in software. Have a poke around in the configuration menu for the router and see if there's a reduced power option and make it full power if possible.
If the source is unwanted interferrence swapping the router will make no difference. It sounds like it's on a low power setting to me though. Make sure it's not sitting right beside a 2.4GHz cordless phone base too.
You may find depending on the router type and settings, that you can disable high-speed Wi-Fi mode and try just a 40MHz channel setting or 802.11g only etc... rather than "n". This will help stop the AP listening to so much radio chatter if the problem is interference.
If it's a power output problem (a faulty rf transistor or just too underpowered) I still wouldn't swap it, I'd get my own higher power one.
Wi-Fi extenders just open another can of worms if it's interference.
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