What started out as a coincidence has become what I'd love this song to be about. We can try to cut this song apart and cross reference everything, but the truth is:

This song reminds me, oddly, of the movie Into the Wild or the main character, Chris McCandless (the movie was based on his life). Here's why:

"Penniless and tired, with your hair grown long
I was looking at you there and your face looked wrong
Memory is a fickle siren song
I didn't understand"

Chris would be penniless as he burned all of his money and donated his savings to charity. His hair is long because he's been homeless (willingly) for a long time. His face in his last ever portrait is really thin and sick looking, as it is theorized that he ate a poisonous seed which caused his death. This song would be sung from the point of view of his sister, Carine McCandless.

"In the gentle light as the morning nears
You don't say a single word of your last two years
Where you were or when you reached the frontier
I didn't understand, no"

I don't think he ever contacted her, and he was gone for two years (and subsequently died in Alaska, definitely a FRONTIER). She didn't understand why he had to do all of this.

"See your rugged hands and a silver knife
Twenty dollars in your hand makes you hold so tight
All the evidence of your vacant life
My brother you were gone"

He would need a silver knife to live in the wild, and I'm sure his hands would have shown some wear-and-tear. No idea what the money would be, as I have never researched his life... I know he worked some odd jobs to get pocket change (like 20 dollars?) for supplies. He's vacant as he owns no fixtures or furniture, and he was finally how he wanted to be (in the wild).

"And you will try to do what you did before
Pull the wool over your eyes
For a week or more
Let your family take you back to your original mind"

The last verse, to me, is that he is trying to cope with his parents' messed up marriage and their bad job in raising him and his sister. He pulls the wool over his eyes, meaning to hoodwink or deceive. He's tricking himself into thinking he wants this. She tells him to let his family take him back to being sane, basically.

"There's nothing I can do
There's nothing I can do
There's nothing I can say
There's nothing I can say
I can say "

There was nothing Carine could have done, as they had no idea where Chris was. Chris died in August after ingesting (theory, of course) poisonous seeds which caused his insides to basically betray him. His body was found in September by some hunters.