A gentle death (was delivering news), Rev 1

[REW]

Silent sentries flank the long hall, (your use of the word sentries here threw me off as I started picturing a military scene.  I suggest keeping it simple here...her folks, her kin, or etc.)
faces averted, their disposition clouded.
In the middle plays the youngest
of her family, fourth maybe the fifth generation;
I am unsure.

They laugh, skip, dance even mere yards
from where she lay dying.

Small, awkward but undaunted as. (a little too wordy here of details)
I cannot hold the inexorable; 
I cross to her room.

Even more are here, older, (I like the warm embers.  I do not think you need as hers lessens as that is already implied).
packed tight, embers that surround
her with their warmth
as hers lessens.

Her husband sits by her head,
his gaze fixed, uncertain, hand
tight on his cane.
Beside is the empty chair
for me.

Her eyes of nearly ninety years
soften as I hold her black hand in my white hand,
and anchor myself.  Relief comes as I hold
her and her husband in my mind, 
all others receding dimly.  I disclose
what she already knows.
They all do.

Sooner than expected 
she becomes still.  Her hand in mine yet,
I tell her husband
"She has passed".
Looking nowhere, eyes empty -- he asks no one
"And now what will I do?"

I reach inside but find
nothing
except that which does not grow back, (I really like those lines about losing a part that doesn't grow back).
leaving me less than I was. 
Forward I must grow -- as forward I turn to go.

The children continue to play (I think this stanza could go to be replaced with more significant details that you mentioned in the backstory.)
in the warm dappled sunlight
filtered through the paneled window.

She has her victory.  Her calm  (This last stanza here doesn't really do it for me.  I am interested in the backstory you provided about being her doctor and the narrator young and white and she old school black from different times.  I suggest adding that here/reworking.)
inscrutable wisdom not lost
but manifest in all those who 
surround, accept and forgive 
and comfort even me.

[quick note:  this was brought to mind by an earlier poem (that seems to have disappeared) that was about giving negative news.]

To me a woman nearly 90 passing away is not a sad thing really as that is a long life, but you seem sad about this.  I imagine the significance is that she, an old and wizened black woman who survived the racist south, trusted a young white doctor. I suppose that doctor wanted to do more because of this (but that wasn't very realistic as medicine can only do so much for a person nearly 90).  I as a reader am interested in having more of that in the poem.  It seems as if this was a very important moment that I think could be played up more.  But that is my view.  I like the subject very much and there are some beautiful moments in this...