[Nerwen]

Sister, dark sister,
Born in the North. -Black, made by the Dwarves of Erebor.
I don't wound but bring sorrow, -The Red Arrow is "a token of war".
You kill but bring mirth. -The Black Arrow killed Smaug. Yay!

Sister, dear sister,
Born in the South. -In Gondor.
You signal: start battle!. -See: "token of war".
I put battle out. -See: killed Smaug. (Though this led to further conflict).

Sister, dark sister,
Fly swiftly away!
You go towards foe, -Towards the Dragon.
I to friends ride away. -To Rohan.

Sister, dear sister,
In scarlet adorned. -With the tip painted red.
You're mightier than the pen -A powerful and instantly comprehensible "message".
I'm mightier than the sword. -Arrow > sword vs. flying enemy.