Firstly I would just like to say I believe you are a very talented writer, especially for your age. Secondly I would like to add that this is probably my favorite piece on this site, so well done.
He was suffocating in lavender.
Great. As people have put before, completely draws you in and now I can't look away.
The scent was a key to opening a floodgate of memories.
I think you could have phrased this differently, It doesn't flow as the rest of the text. Takeaway the first a or the second and make floodgate plural. I just sounds too literal.
It was his worst nightmare relived, revived, and made a hundred times more painful. It was the aroma of guilt heavy on his tongue, pressing on his heart. It was her.
Good sentence structure, makes me want to read on.
Alex Papoulis came into his dead sister’s room with six things—two laundry baskets, a sponge, a bucket of soapy water, Lysol, and a mission.
Okay, there's a lot of information to take in. We find out his name, and the fact he has a dead sister. When you go to list what he brings it distracts the reader from the main focus, the fact he has a dead sister. Also I have no idea what Lysol is, maybe do something more widely none like bleach. And that last bit "a mission" please pleas please take it out!!! It spoils the previous text, everything is deep and emotional then you add in a cliche line that is totally out of character.
That was the problem with death. It was so inconvenient. It could take away everything that you hold dear, take away everyone close to your heart, but still leave a mess behind for you to clean up. Death had a scent that was once faintly soothing turned into something repugnant. Death had Alex ransacking his sister’s closet and hamper, stuffing every article of clothing she owned into a laundry basket, and spraying the empty space with Lysol, just for good measure. Death had Alex on his hands and knees, scrubbing away at her floorboards with safe citrus-scented soap in attempt to lift some of the reminder off the floor. And Death had him frozen at the window, looking at a mid-day sky that was the color of false happiness.
This is good, I really get a picture of him moving from each place in her room, moving to the window.
“Could it at least be a gloomy, dismal afternoon?” he muttered under his breath as he attacked her bedspread, tossing it into the basket. “Of course not. It has to be a beautiful day, complete with stupid chirping birds and laughing children.” He scowled at the injustice of it all. The world outside should fit his mood. Instead, it kept on going after the accident as if nothing had happened.
But Alex knew better. Everything had happened.
Nice speech but a little to formal, even though he would be grieving he may not use as much description and elect language as you have written. Secondly you repeat the sentence structure from the second paragraph, with "It was her". I don't know if this was deliberate or not.
Nice ending, good writing and description. Other than the problems above I would say a fantastic and moving piece.
PM me if you have any other questions
