Back at her apartment, she changed into a t-shirt and panties. I sat around in my underwear. When she sat by me on the couch, I decided it was time for more practice and to see what I could get away with. I put my arms around her in a nice hug and started kissing. She responded eagerly. As we were kissing, I decided to gently caress her boob. At first, she stiffened and stopped. Then went right back to kissing while I was obviously massaging her boob. Her kissing became more intense. I decided to try for more and slipped my hand under her shirt and went right to her bare boobs. I liked the feel of her nipple, the warm round breasts and her breathing increased, but she seemed to like it. I sure did.
When we sat down, I hadn't thought much about us being in underwear; it wasn't uncommon, but now I was very aware of it. I had my hand up her shirt exposing lots of skin, and there were her panties. Suddenly it hit me. This is my sister! It felt naughty, but I couldn't stop. I pulled up her shirt and started to kiss her nipples. Her panties were getting wet, the tip of my dick was leaking. She was now caressing my balls and my dick. She had my shorts pulled down. I said I forgot you're my sister, we can't do this. She said, "Don't Stop".
I wasn't strong enough to stop. I grabbed her panties and pulled them off. She lay back on the couch and spread her legs. I kissed those lovely legs and worked my way up. Suddenly I was there, actually licking my sister's pussy! I would have kept this up forever, but she grabbed my head and pulled me up to her, kissing me more passionately. I was on top of her now, putting my dick in her, in and out. Then I felt the orgasm, and it went through me. I tensed up and almost screamed, "OH MY GODDDDDDD" and then felt the rush of pussy juice telling me she was Cumming too.
This was an overwhelming experience for us. I asked if we were wrong to do this, even though it felt so right. She told me that since I was her boyfriend that I was supposed to fuck her. So I held her gently, kissed a lot, then went to bed and fucked her again.