Watch interviews and documentaries. I'm working on a story rn starring an autistic lead, as I'm not autistic, I'm watching videos on ASD, interviews with autistic people, xyz.
The truth is, you can't experience everything, so you'll probably have to write some things you don't know about, thanks to the internet though, that's very possible. An annoying sense of humor might be hard to research as everyone has a different opinion on that, but you could look up videos on annoying things siblings do, because most people have those ideas in common, and translate that over to you character. As far as trauma is concerned, there are a whole lot of interviews, videos and documentaries on these things, but be forewarned that it might be very hard on YOU emotionally, to watch these.