Okay, I have never admitted this one anywhere but.. I have a fantasy involving my brother. He's 4 years older than me.
Some background on me. I'm trans and came out early to my family. Long story short, I began living my life as a girl from age 7, started testosterone blockers at 12 and estrogen a year and a half later. I never experienced male puberty and my body developed as other girls do. By my teens I became as my big sister told me, "a heartbreaker", a very attractive girl.
One time when my brother was 18 I came home from school early and caught him fapping to trans porn. He was so embarrassed. I had never seen him so vulnerable. He had always been my big brother, my protector. I asked him gently if this was a regular thing for him or the first time he looked at trans porn.
He was flustered, blushing and blurted out. "Look! I watched my little bro become my little sis and grow up to become a hot girl, wtf am I supposed to do?!?!"
I just gave him a hug and told him it was okay and I wouldn't tell anyone. I could feel his cock twitch through the thin fabric of my dress as it pressed up against my tummy. But that was it. We never spoke of it again.
But... ever since that day I have fantasized what would have happened had things gone further..