Wifi extenders would increase signal range if you place it in the right spot, but I'm not sure if they would increase speed in your situation. If you're limited to making changes on your end, then there's not a lot you can do..
The cheapest solution would be to use a USB extension cord, so you sort of get the same thing as an antenna on a wire. Plug the wifi adaptor into the USB extension cord and place the adaptor closer to your uncle's house. If the computer is near any windows facing your uncle's house, I'd try to put the wifi adaptor at the window, it should interfere less than a wooden/brick wall.
Really, it's sort of hard to say definitely what to replace or change. I assume you're using wireless N, but I have no idea what speeds or capabilities your devices support, such as the USB adaptor you have or the modem or router at your uncle's house is working at. Stuff like antenna type (slow but cheap and compact 1x1? high end 3x3?) and frequency (2.4GHz is better for range and wall penetration than 5GHz) can matter a lot. If there's a lot of other wifi networks in the same area, there could also be congestion and interference.
For example, if the wifi signal is broadcast with a device using 2x2 antennas, getting a new wifi adaptor that supports 3x3 won't make a difference if your current adaptor already handles 2x2. And if the wifi signal is wireless N only, getting that Gigabyte PCI-E wifi card which supports wireless AC might not make a huge difference, although it is compatible with wireless N and has 2x2 antennas.
This document is a little long, but it goes into more detail about wireless N networking and how to optimize it.
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