Here's the revised version. They're minor changes, but I think it helps a lot.
Endless Night
She was the light, always surrounded by the void
She was the single mother who raised two healthy boys
He was the dark, engulfing everything he touched
He was the moment she realized she'd had enough
I was just a boy and I didn't understand
The power that he held with the motion of his hand
And my brother came to me in the dead of night
Said that he couldn't sleep when he heard his mama cry
And we were just shadows of the light she had harnessed
We were the only thing that kept her from the darkness
Because her life had become an endless night
But she told us that we were the only shred of light
It slowly got worse but she seemed fine for awhile
She learned to hide her pain behind a broken smile
She'd wave from the window, watching me and him play
She cherished every moment of every single day
The smile soon turned into a blank, lifeless stare
She had lost so much weight and began to lose her hair
My brother didn't get it and I just couldn't say it
But I knew I was watching her slowly decaying
As time slipped by she grew too weak to fight back
And the light that once surrounded her began fading to black
To most it would seem like the darkest kind of horror
But she seemed so calm when the reaper stood before her
And we were just shadows of the light that she had harnessed
We were the only thing that kept her from the darkness
Because her whole life had become an endless night
But she told us that we were the only shred of light
One cloudy morning, she got up and called my dad
Asked him to pick us up, because this time it was bad
My dad heard her screams before he opened the door
He asked me how long this had been going on for
I told him the truth and I couldn't help but cry
"What if one day he hits her so hard that she dies?"
She knew she'd never give him that kind of power
So she write me a letter in her final hour
"It's gonna be hard to live without a mother,
But be Mama's big boy and take care of your brother."
"Tell your dad that I love him and I always did,
Even when we separated and he took you kids."
"I know you'll be okay and I know you'll go far,
And if you ever miss me, just look up at the stars."
"Cause whenever you need me, I'll always be there,
And I'll always be that light whenever you get scared."