With my real sister

I fantasise and masturbate to elder sister. It was around 8 years ago when I was in school. My sister was a really beautiful woman. She was in her 20s. She used to earn well, dress well and looked really amazing then. We used to share the same bed as we only had one room to share. We had family of four- parents and we two siblings. It was a normal brother sister relationship, until something happened. We used to sleep next to each other. One day, while sleeping, she asked me to give her a back massage. It was just a regular back massage. When I started doing so, I started enjoying touching her. It was the first time I started to feel lusty about her. I dont know why I started to feel such way about my real sister. Days passed by and it was almost a daily thing after going to bed. I used to massage her daily. My feeling started to get even more lustier. One night, while giving massage, she held my hand and took it to her vagina. I tried to resist, but she took it anyways. She might have thought I was asleep, or maybe not, but I was not asleep. I could feel everything. I still remember her juicy hairy pussy. At first, it felt weird because it was my first ever experience touching a vagina, but I started enjoying later. She used my fingers to masturbate. I can tell she was really horny. I was enjoying every bit of it. It was really beautiful. Another day, she took my hands to her boobs. Then everyday, after going to bed, we used to involve in different activities, with her thinking I was asleep (this is what I assume). Some days we kissed each other, somedays we cuddled sensually, and some days we touched every parts of each others’ body. Basically everything a couple would do (except sex). She touching my dick felt heavenly. All these things were the best feelings for me, which I used to experience every night. It continued for almost 2 years. Since then, I have been fantasising about my sister. Now it has been really long since that time, but I cant stop thinking about all those moments. I think about her every time I masturbate. I dont know if she remembers but I cant forget those moments. I fantasise and masturbate to my sister. Am I wrong and sinful for this?