Evaporative Cooling of Group Beliefs in the Mormon Faith
This can be explained pretty naturally with evaporative cooling of group beliefs.
Ironically, despite my serious long-term relationship with another man, my more relevant experience here is still leaving Mormonism. When I left, I spent a few weeks posting on /r/exmormon. It was a useful place for me to process the sheer pain and uncertainty of leaving. The incentive pattern there was crystal-clear: when I posted things more critical of Mormonism, experiences like the church leader who told me to send him questions and then turned it into "Here's why you're a prideful apostate, I got showered in attention and sympathy. When I posted things that presented a sympathetic view of Mormons in my life, but still validated a leaving narrative, I got moderate levels of attention. But less so. When I posted about how much I cared about and missed parts of it, I met with mostly a shrug.
Who's going to stay in an environment like that? For the most part, it's the people who have ongoing negative contact with it—who can't just neatly extricate themselves. People who lost spouses in their faith transition, or became estranged from their children, or live in Utah and are surrounded by Mormonism no matter where they go. People immersed in it, who need an escape. People who find themselves still fascinated by the faith, and immerse themselves in scholarship about it. A very specific slice of ex-Mormons, basically. Like, you're just not going to see Jacinda Ardern hang around there, or Kyrsten Sinema. They've got other things to do.
So someone like me goes there, and I process for a month or few, and I get a weird pattern of validation where the worse anecdotes I have the more sympathy I get, and then I leave and get on with my life. And someone with a much worse experience goes there, and stays there, and stands as a building block of its local culture in a way I never really would.
Their experiences were completely incongruous with my own, for what it's worth. Some people have really, seriously miserable experiences leaving Mormonism. I did not. My family and friends were as supportive and loving as I could possibly hope, and only a handful of older people (granted, a few I really did care deeply about, including my mission president who was very much a father figure in my life) really became incapable of treating me as anything other than a damned soul in need of salvation. And even those were kind to me, in their way, just misguided, and mostly just left me with humorous anecdotes like trying to cast a devil out of me. I was primed for something awful to potentially happen, for a long-hidden side of otherwise innocuous people to come out... and it turns out everyone was pretty much exactly who I expected them to be.
But. I've also met people I deeply trust and respect who tell me their own experiences have been much worse, and provide credible details to that regard. They're not imagining pain. They're not inventing the lifelong relationships of trust that break down, or the wedges that spring up between loved ones.
Likewise, I spend very little time immersed in gay or LGBT culture. It just isn't for me. I self-select away from that stuff. My boyfriend is passionate about his (entirely unrelated) career and spends the great majority of his time on things that have nothing to do with being gay. In my life—in-person at least; I've gotten a handful of hate messages from hanging around online—I've never faced the slightest bit of visible harm or trauma from openly being in a gay relationship, or from 'coming out' about my feelings for men. As a result, I have very little need for any sort of 'gay community', and am unlikely to contribute much of anything to 'gay culture'.
On the other hand, I've met people with heartbreaking stories. One that stands out is a guy I went on a date with whose family kicked him out for being gay. He grew up in an evangelical Christian family in the South, and... stop me if you've heard this one before. He wasn't the sort to invent drama, and he was still a believing Christian who wouldn't look out of place in a bible college. Another is my previous significant other, a queer nonbinary woman whose Catholic parents made their life miserable in a number of (verifiable) ways, and who was actively embedded in the 'queer' scene. The sort of person who becomes able to define and contribute to group culture. They really did have, and need, a 'family of choice', and that defined much of their thought on the issue.
To swing all of this back around to your point, my simple response is that what LGBT people are like and what the LGBT movement is like are very different. This is not a knock on the movement or the people, it's a result of predictable population dynamics. The people who need it more are likely to be those with worse experiences related to it, and the people who contribute to it more are likely to be those who need it more. More, the incentive structure in those communities is inevitably centered around reassuring and amplifying the voices of the ones who validate that narrative the most.
This can happen without anybody falsifying their own experiences or exaggerating consequences they've faced (though exaggeration is also possible as a natural result of the incentives). Then it turns around, and presents what collectively becomes a much more exaggerated picture as truth to randoms looking in. Those who have experiences incongruous with it, not finding a need for the community, move on and become the Nate Silvers and Peter Thiels of the gay world. Those for whom it rings true stay and perpetuate it, and at times speak it into being, as with your experience with your sister-in-law.
Narratives are powerful, and so are group dynamics. All that needs to happen for the trend you observe to take over is for a community to center around the experiences of those who have faced the most pain within it. I'm still unsure of the correct response, but I do think the process itself is pretty straightforward.