Warning: *Very* long review.
Overall, not a bad job, and an interesting story, but I think it could use some polishing.
I'll do criticism first, and what I liked second.

1)“Icy crystals of snow practically floated from the sky's clouds”
--Why “practically”? I think the word is unnecessary. Weren't they just floating?

2) “Oh how the snow looked so tender to touch, how she longs to stroke the piles with her fingers, to throw herself in it, to spin round and round and not stop a second early before she became too dizzy to focus.”
--You switch between past and present tense, which is confusing. I'd suggest making it all past tense, as the rest of the story is this way.

3) “and slinking through a small crack in the door, just big enough to let her through.”
There's something wrong here; there needs to be a verb following the first part of this phrase, otherwise it doesn't make sense.

4) “as she heard distant footfalls” --do you mean “footsteps”? I've never heard “footfalls” before, but that doesn't mean it's wrong.

5) “peeking sneakily inside” --I think I'd change this to “furtively peaking inside”. It just sounds better, in my opinion.

6) “just in time to see her Mommy's profile as the woman ducked into the bathroom, grabbing something in a pantry.” --Two things: “Mommy”, when preceded by “her”, should not, I think, be capitalised, as it is not a proper noun when used with a possessive. Second, a pantry in a bathroom? Do you mean a medicine cabinet?

7) “Pushing the door fully open, she sneaked inside” --Both “sneaked” and “snuck” are correct, but personally I think “snuck” sounds better.

8) “That's when she saw it.” --It is too ambiguous. Saw what? Maybe just leave it at “saw”; I think it would give it a good sense of mystery.

9) dropping the card into a gift bag that stood on the mattress.” --Gift bags don't stand. At least I don't think it sounds right. Maybe “lay” or “sat”? That could just be me though?

10) “short pieces falling long enough to her night gown collar.” --Long enough for what? It's like there's a word missing or something.

11) “The scene in front of her tore her heart to pieces. She looked harder at the tip of the envelope in the bag.” Her, who? The mother or the girl? There needs to be specification here.

12) Several comments about the italicised portion.
a) “inside of an ebony car” --take out the “of”; it doesn't make sense.
b) I'd suggest not saying “And the teenager never looked back.” Instead, put “girl”. Her age has already been mentioned, so she'd just be a “girl”, in this case, in my view.
c) “Where was her sister going?
But, why was she going?”
-- “But” doesn't seem to belong here; it's not in conflict or juxtaposition with the first statement. Take it out, I'd suggest.
d) I think it the flashback starts confusingly, as the reader doesn't know what this is. I think adding “She remembered” to “Her sister.” would be less jarring, and I think putting the memory in present tense would be especially effective.

13) “By the time her mere flashback was over, the angelic girl”
--I don't think you should describer her as “angelic”. The scene isn't about the girl's virtues; therefor the adjective seems out of place. “stiffling” should have only one f, by the way. And I'd leave out “rosy”; it just seems unnecessary.

14) “Why are you crying, my little, pumpkin-seed?” the punctuation here seemed strange. Should there really be a comma between little and pumpkin?

15) “her mom begun” -- began.

16) “If Mommy's sad, it's okay. All of us get sad from time to time, yet, that doesn't mean that anyone else has to get sad, also.” I think there are too many commas here and they interfere with the flow, plus it doesn't really sound the way someone would speak.

17) “She hugged the blonde to her bosom,” --I think there have already been too many references to her hair colour, and also eye colour.

18) “she said in a confirming tone, sweet, though” --I think this would better read as “he said in a comforting tone.” My first assumption would be that a confirming tone would be harsh anyway, so the the “sweet, though” seems odd. I think “comforting” would be a better verb, too.

19) “They embraced for what felt like nothing, yet in reality, was forever: --That doesn't seem to make sense. I'm sure they didn't embrace for “forever”, and it felt like *nothing*?

20) “But, the taste was erase” -you mean “eraseD”.

21) “And its my turn” --it's.

Okay. Now what I liked.

“The atmosphere in the room buzzed at the fidgeting silence, becoming almost frighteningly apprehensive.” --I love the combination of “fidgeting” with “silence”. Very cool way of putting it.

I like how first the water had never tasted so pure and then it had never tasted so tainted. Simple, effective.

I like the tension when the girl is hiding under the blankets. I was expecting a different type of story; one where the mother is angry at being surprised and watched. So it was a nice surprise. Good bittersweet story.

This was really good. It was interesting, though the plotline seemed a little strange at times. The snow bit at the beginning didn't really make sense to me, but thinking about it now, i suppose it was necessary. The piece was really quite good, though. The emotions portrayed were very realistic. Good job.
~BJ