Linda called (my older Sis)... You must come soon. <<--I don't think you need the bracketed (my older sis)
but I could not for two more days...
The day before, they both had joked all night,
and she Mom was ready to go home.
The doctor came this morning, suddenly,
We've done all that we can.

Scars from pneumonia one too many times
had left her feeble lungs unable
to enrich the blood that fed that heart
with which she which reached out to so many
in her single years...
when she grew up, became a Mom
and Grandmom all at once...
to more than you could count.  <<--I'm not sure you need to expand

When Linda talked to me, she Linda said,  <<--don't think you need to say "when she talked to me"  seems self evident
After the doctor left, it was as though
the darkness filled the room, and all the joy
evaporated suddenly. You must come home.

So two days later, I called Art
not long before I got aboard,
I called Art:
This is the end; she's holding on.
I'm not sure you will make it, though.
He was so close to Mom -- the last
to live with her alone, protecting her.

While I was in the plane, Mom's sons
and daughters, friends, grandchildren,
siblings, nieces, nephews, gathered there with her
and one by one conversed with her, and laughed
and cried... some with her, some alone, outside.
A lot of hugs... and kisses on her skinny lips.
I was the only one
not there.
Tacoma is so far away.

Chicago now, and I call Art again.
I have to go... no time to talk. She's dying.... click.
Oh, no! She'd told them she would wait
'til I got there. It hurts that she could not,
but I don't want her in that lingering pain.
I let her go, but I feel empty, helpless.
I'm the only one
not there.
There's still so far to go.

The moments just before I'd before I called,
my niece Elise, who'd lain with Mom
right up against her chest, could feel
her struggling, holding on,
like she was waiting for my voice
to say Goodbye before she flew
ahead of us to all that God's prepared.

I think it was Elise who whispered then,
I wish someone would talk like Donald Duck.
She'd know that Uncle Dan is here.
And suddenly her husband Matt quacked out
familiar words... the telephone began to ring,
Art picked it up, I have to go, no time to talk...
and Mom let go.
I was no more the only one
not there...  <<--I know what you're trying to say here but I wonder if there is more impact if you left the verse on "and Mom let go"
though I was still so far away....

The day is coming I'll see her again
and I'm not sure
that I have far to waddle on.  <<--I'm not keen on this line, it seems to bring in a humour that doesn't work.  Maybe just have "the day is coming when I'll see her again.  It won't be long"