Long post, strap in for a boat load of background context.
For this story I'll call my sister Gel and her friend Agar. Agar is my age and Gel is two years older than me.
When I was in middle school I was a bit transphobic, though more so in an internalized way. I was in a friend group with a lot of trans and queer kids, and I generally wasn't bad toward people who were lgbtq+. At this point in time both Gel and Agar identified as boys, and they weren't friends yet. I was friends with Agar, and I would talk about how much I enjoyed the people at my church, some about how Christian I was but generally just in the way that I felt more enlightened, and invited her to my church once.
In 9th grade I was still friends with Agar, who I told mean crap about a boy who had a crush on me because he was a bit weird. Agar then told Gel, who used that to try to get me into a lot of trouble at home. At the time I didn't realize it was Agar who had done that.
Close to the end of my 9th grade year I came out as lesbian, and ended up telling both Agar and Gel.
Some time after that both Agar and Gel desisted, they never started anything medical for it to be accurately called detransitioning. I also fell out of touch with Agar for a while. Gel is still queer, I don't know about Agar.
During 11th grade I asked Agar if they could set me up with any girl they might know. Nothing really came of that, and we didn't continue to text very much.
As time went on I came to know myself as a straight man, though I'm not sure if either Agar or Gel know about this.
This year, in 12th grade Gel decided that because I didn't want to have another argument with her about random bs that she was angry about she would out me right then and there to our parents. Apparently Agar had told her about me wanting to get set up with a girl, and she used that information. I was able to pretend and convince everyone that she was just talking about some phase I had. There were still repercussions, but I didn't get to act like those were an issue.
I don't really know what I did to make Agar so malicious against me, maybe it was the things I mentioned from middle school.
This summer Gel and Agar have become best friends. It has been so upsetting to hear Agar's name again and again. Today Gel invited Agar over to our house.
I tried telling a teacher that I'm close with about it, but she cares about Agar, so she just corrected me on Agar's pronouns, and when I told her how Agar had desisted she didn't say anything else. I told another friend, but I guess she didn't know how to respond. There probably isn't really a right way to respond in this situation.
Edit: fixed wording