Uncomfortable experience with my brother?

Hi. I’m thirteen and I live with my mother and two brothers, one older one younger (three boys). We were living with our step dad, but he moved out due to various problems. We’ve had to move quite a lot since then because my mother keeps getting told that I’m too smart or that I’d be much better off in a private school. We can’t afford a private school but I think she means to have me try for a scholarship next year.
Anyway, this is about my older brother Ryan. Ryan and I used to get along great. Last year, when our step dad was living with us, he started molesting Ryan. It lasted until my mother found out and kicked him out, which was quite a long time. Ryan is a year older than me and neither of us has hit puberty yet. Our mum used to work late nights and we’d make our stepdad keep us company in our room till we fell asleep. He’d wait till we were asleep then go after Ryan. One night I pretended to be asleep and heard him caressing Ryan’s legs under the covers. He’d do this every night, then he started getting bolder. He’d call Ryan to his room when mum wasn’t home and start groping him all over. Then he’d start kissing his body and feet. One time we were watching TV and he came in and told Ryan to take his trousers off. Then he started cherishing Ryan’s thighs, before proceeding to lick them all over. He never touched me or Jack. Ryan was the sole focus of his abuse. When he left it was like a huge cloud was lifted off our heads. But then I started having psychological issues which have bothered me ever since. I’m very grateful that that pervert never touched me and Jack, but I was really affected by what he did to my older brother. I never saw Ryan the same again after our step dad left. I felt like he had something I didn’t. Like he was…I dunno riper than me or something. It was no more than just an annoying thought that I would brush off irritably at first. But then it kept coming back again and again. Before long I realized it had gotten so strong that I could no longer ignore it. Then it turned into an inferiority complex, which really started to worry me. I knew I was smarter than Ryan but I just felt very inferior to him and didn’t know why. It was horrible. Whenever he stood next to me, I’d unconsciously try to stand taller to match him. I started to get angry at him for the smallest things he did. Little things that I used to shrug off really upset me and even made me cry. Whenever our mum would buy him something, I’d get really jealous even if I didn’t want it. I couldn’t stay in the same room with him without feeling inferior and always ended up starting a fight. Then last week, Ryan barged into my room and asked me if I’d eaten his carrots. I shouted at him to get out of my room and that I hadn’t touched his accursed carrots but he thought I was lying so he started hitting me. When I fought back he pinned me to the ground so I couldn’t move then put his foot in my face. This made me feel so inferior and angry I can’t even explain. It was like he’d taken all my previous feelings and magnified them a thousand-fold. It was a really terrible feeling. I screamed at him to get off, –yea, even BEGGED him, cos what he was doing was really hurting me, I’ve never felt so angry in my life, I felt like I was on fire. It was like being tied up and burned alive. It was like he had opened up a dam of rage but there was nowhere for that rage to go. He refused to get off me. Then I got an erection.
Let me clarify something, I’m not worried that I might be gay, I know I’m not. Nor am I looking for sympathy or reassurance. All I want to know is why my body reacted the way it did that day. That’s all. Ryan had it at first hand but he was not affected by my step dad’s behaviour as far as I can tell. Why was I affected so deeply?
If you feel you don’t understand what I’m asking please don’t answer just for the sake of answering. I’m very intelligent. And all I’m looking for right now is an intelligent answer.
Thanks for reading.

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