I can't really say 100%, but check your gfx card settings. Might be something as simple as an antialias thing. But check your card settings and compare with your brothers. If all else fails, try to uninstall and reinstall the game. Been a while since I've played it due to lack of development for now (of which I do understand though), I also would love to see this game take off more as it has so much potential.
I and mostly all the others here play Real World Fly Fishing. It's a bit more simplistic, but honestly you can't beat it for the money. Try the demo, see what you think. The full version gets you not only the game, but you'll have access to literally almost 340 (and counting) fish species, freshwater and saltwater, and almost 1,500 custom outings (and also still counting). Check out the forum above, both the RWFF forum and RWFF Development forum to get an idea. But believe me here, it's a hell of a deal. I don't fly fish in RL myself either but I LOVE that game.