I read this online, on a site for SFF short stories, at some point between mid-2016 and May 2017. I am confident it was published within the last 3-5 years or so. I don't know how many words it was. It seemed like a quick read, but obviously a lot happens in it.
It seems like it should be easy to find, but unfortunately all the keywords I try seem to lead to either bird pollution stories or "Is [insert character from popular media franchise] a sociopath??"-type clickbait articles, so I'm stuck.
The protagonist comes from a group of xenophobic humans who left to colonize their own planet after deciding the main 'federation' of planets was becoming too alien-friendly. The new-planet-colonization was supposed to completed before the protagonist was ever born, but they're an adult now and it's still going horribly; the air is thin, the water is bad, and this strange goop covers every corner of this planet, infecting all the insects/birds/plants the colonists release to try and bio-engineer the planet.
The protagonist is a sociopath, the first ever born on their colony. (They're also nonbinary and use the pronoun 'they', which is why I use it here, but the story is in 1st person POV so this fact only comes up once.) Almost everyone on their planet views them with fear and suspicion, seeing them as a violent criminal who is liable to snap at any moment.
The colony is very low-crime in general; there's only one person on the planet who ever committed a serious crime, a researcher who murdered a fellow researcher some years ago, refused to explain her motivations, and has been kept in solitary confinement ever since. (The protagonist has committed a lot of petty crimes--arson, killing small animals, running away and jumping into the weird goo to see what would happen--but is adamant that they'd never commit murder because they don't want to end up in solitary confinement like her.)
The main plot is that the protagonist's sister, possibly the only person on the planet who really loves them, is running for governor of their small colony on a platform of being more aggressive in getting to the bottom of why their bio-engineering efforts are failing. The protagonist's brother, a bio-engineering scientist who resents the protagonist because he was initially blamed for the protagonist's arsons back when they were children, starts acting unusually friendly towards the protagonist around this same time. This gets the protagonist suspicious, and they start looking into their brother's weird behavior.
The protagonist visits the woman in solitary confinement, who also views him with suspicion, and then visits their brother's research labs. There, the brother shows sick-looking and healthy birds side-by-side--and then tells them the healthy birds, not the sick ones, are the one that were released outside and went into the goo. The protag realizes that the goo on the planet is actually a sentient single-cell organism that binds with all the living things on this planet to make them able to withstand its environment; the older colonists, being huge xenophobes, couldn't handle this fact and have been covering it up for a long time now. That's why the researcher committed murder. The sister's desire to look deeper into the issue is getting support from the younger generation, so she's now in grave danger.
The protagonist returns to their sister to tell her the news, but instead finds her tied up to a booby-trap that's about to go burn her to death: the brother, actually a hardcore believer in the conspiracy, lured the protagonist back here to kill them both. Everyone else in the colony will believe that the sociopath protagonist snapped and decided to murder their own sister and themself.
The protagonist manages to free her, but before they can escape entirely the booby-trap goes off. They wake up in the hospital, the protag in fine condition but the sister near death and suffering. They realize the fact that they jumped in the goo as a child, accidentally melding with the symbiote, is the reason their condition is so much better than hers. So, when no one else is around, they secretly carry their sister out of the hospital, apologize to her for killing her pet bird when she was a child, and infect her with the same symbiote that they infected themself with by jumping into the goo pile back in the day--leaving their sister's fate up to the same alien that saved them.
Some more details I remember:
- No important character has a name--it's 'I', 'my sister', 'my brother', etc.
- The protagonist's sister is a lesbian. At one point, the protagonist, upset that the sister is paying more attention to her girlfriend than them, starts maliciously trying to convince the sister that her girlfriend is cheating on her.
- Right before the booby trap goes off, the protag realizes their brother still hates them for how, back before the colony realized they were diagnosed as a sociopath, they framed him for the fires they set and acted like a perfect angelic child. They're genuinely shocked by this, as they can't imagine caring enough about anyone else to hold a grudge for that long.
- The protagonist remarks at one point that they do love their sister--they love her the same way they'd love a favorite, irreplaceable toy. They understand that this upsets people to hear, but not why.
- At one point the protagonist deliberately burns one of their sister's campaign posters and, when caught by colony security, acts like it was just some uncontrollable urge they had. It's later revealed that they did this because they knew punishment for a minor crime like that involves having a location tracker fitted; they suspected they were going to be 'disappeared' by people involved in the conspiracy and wanted to make themselves traceable just in case.
- The protagonist works as the head funeral director/cremator/something like that.
- The one single scene where the protagonist's gender comes up is a flashback to when they were a child: they're on trial to determine whether they should be locked away as a danger to the community. The person arguing in favor talks about how "it's a danger to us all" and the protag responds "I'm not an it, I'm a they."