The windows were squares of blackness
, thin barriers to hold back that quiet blank vastness
that she couldn't stand.
First, I really like the phrase "quiet blank vastness". Your two -ness nouns in a row here stand out a bit more than you probably intended, though, and the ending "that she couldn't stand" seems arbitrarily tacked-on; it doesn't flow with the rest of the sentence.
the teenage waitress ... sat
down a pitcher of coffee.
"Sat down" is a thing you did; "set down" is a thing you did to something else.
At this rate she'd be spinning lines about love-lorn hearts and drinking alone within--
Ah. Grace is here.
Nice moment. "Spinning lines" is a bit of an awkward phrase in my opinion, though.
On the surface, she was unimpressive enough.
You say this, and then follow up a full paragraph describing how good she looks.
Diane couldn't help noticing that Grace probably looked better than she did herself, at almost twenty years older.
This sounds like Diane is the one who is older (because of the proximity of "herself", which makes it kind of a weird sentence. But, we find out next paragraph that actually the opposite is true. When there's a 20-year difference between characters' ages, it's important to make sure it's clear which one is which.
the rumble of her stomach got the better of her. She ordered a too-big plate of greasy breakfast food
It's not clear who "she" is; the last subject you have is the waitress so that's obviously not right, but then before that it's Grace--which we find out later is also incorrect. There's nothing wrong with "the rumble of Diane's stomach".
Okay this is kind of weird given how much time we just spent ogling Grace's legs.
her inner twelve-year-old actually had to restrain a squee when that giant brass-feathered condor swooped down on her a few days ago.
That is some
"Younger than what, exactly? Western civilization?"
Lol. I like it, though Diane isn't showing as much discomfort with apparently being a demigod as I would have expected.
She'd do it, too. Diane could already feel herself trying to slide off the booth, to throw herself down and beg for forgiveness. All she felt was an icy spiral of shame and fear. She would give anything to make Grace happy, to erase that pain, to see her smile.
This is very well-done. We're seeing the strength of Grace's personality in a way that didn't come through in her introduction, because that was exposition, whereas this is demonstration.
the small bundle
Watch your tenses.
with an oddly curved shape
and an apple-shaped
The repetetiveness detracts from the otherwise very elegant description here.
Turn the blade on yourself tonight, and the ties of fate and lineage will be severed.
I liked this snippet a lot! Your prose flows very well and you have some great descriptive moments. Both characters come off very well and you do a really good job with Diane's ambivalence. It's a great first contribution; I hope you're planning to stick around for more!