I think that this depends to some extent on where you think that your parents come down in terms of their political beliefs. Do they agree with your side, your brother's side, or neither. In general, though, I would emphasize a couple of things:
- I would emphasize how and why I think that my side is in the right. I would talk about why I think that helping my side is important. This establishes a reason for me being out there shooting people.
- I would then emphasize how sad I am and how much I did not know that my enemy was actually my brother. I would tell them how I never saw the enemy sniper until he was already dead. I might say that I would rather have died than kill him, but I did not know what was going on.
Overall, I would make the letter sound very sad and apologetic.