Look at these:
WHR-HP-G54 - Access Point
WLI-TX4-G54HP - Ethernet Converter
The above buffalo products are supposed to have a higher output power and have a connection for an external antenna. You can look at other companies with bridging products too, but it seems that they have lost interest in this type of thing...
As for external antennas, I linked a site above that has some. Try this:
2.4 GHz 14 dBi Backfire Wireless LAN Antenna It has a relatively wide beamwidth, making it easier to aim.
You will also need a RP-SMA Plug connector plug cable (at one end) and a N-Male connector (at the other end) to connect the buffalo box to the antenna.
Verify this cable will work! You should be able to find longer cables, but not recommended, mount the buffalo box as close as possible to the antenna and run ethernet cable to it. It will need to be waterproofed, unless you plan to take it down on a daily basis (or it doesn't rain where you are)
There are several options to set up your link. You may be able to get your link to work directly to your cousin's but I wouldn't bet on it. Try setting up a simple link first (single point to point) Then if that works, go for the point to point to point linkup.
I think the rough cost of the the above is just under $200 (2 buffalo boxes, 2 antenna, 2 cables) this will give you a single point to point link. Double that to add the next link.
Since you are roughly halving the link distance by adding a central point; another option is to add an
omni-directional antenna at the center link location and to use WDS to connect the dots. This setup would be a little cheaper as you would end up with 1 less buffalo box.
Either way start with a single link and play around. Get the hardware working in the same room first (configure the router, etc) That way you can focus on pointing antenna instead. First try the link from your house to your friends, then move the equipment from your house to your cousins and try that link.
You could try building your own antenna's; --there are many sites on the web and it doesn't look that hard, but it adds some risk or potential frustration.