Quick disclaimer – English is nor my first language so bear with me.
This story starts almost 15 years ago, and has a bit of a build up.
My mom and dad had recently split up, and my dad started dating a new woman. She had a daughter from a previous relationship that was just a year younger than me. At the time we were just 11 and 10 years old, so nothing sexual happened for a while still. We just quickly became good friends and loved to hang out with each other.
Fast forward 5 years. By now my dad have bought a house with his new gf and they married making us step siblings. Since my dads new house is very close to my moms I would spend much time with them, and over the time became very close with my step sister. At the time we're 15 and 16 years old, and gone from being good friends to being very flirty with each other. Still nothing sexual – my step sister would always have a boyfriend, but she would flirt and tease me mercilessly whenever her bf was not around. It drove my pubescent mind crazy and I spent most of my time in my dads house with a semi in my pants or a raging boner in hand behind a locked bathroom door. I would masturbate every night in bed knowing she was lying just inches away from me on the other side of the wall, and I think she knew what I was doing as the bed was squeaking badly with my rapid hand movement.