The first time I got a blowjob from my sister

I'm not expert story writer here, but I've been reading stories on here for long enough and I feel it's probably time I make a contribution of my own. I'd like to think I do an okay job at this but if I don't by all means hurl abuse at me in the comments section.

Anyway, a bit of background information here - my (step) sister was 22 at the and I was 17, I had a huge crush on her. She had such a perfect fat white ass, I used to watch it jiggle as she walked everywhere, she had a fondness for very see-thru leggings (yoga pants), on a good day you could see a g string through them, it was bliss. She was quite petite at about 5'4", but with great tits and nice curves, short blonde hair (think P!nk, shaved sides etc, blonde etc).
Anyway, one day a couple of summers' ago we were walking to a shopping complex. I needed a haircut and she needed something else. It was a great day, she had a string top on that you could clearly see her bra through, and her tits were in plain view from above (I am considerably taller than her, so it was easy to get a look at them)and she was in sheer black leggings, her ass was perfect as ever and I could see her g string through them from a mile off when the sun shone on them.
On our way home I was messing around with my phone and eventually started taking photos of her ass as she walked in front of me (in hindsight, a bit pervy of me, I was young and horny and stupid). Anyway, she turned around to ask me something after a few minutes and presumably caught me taking snaps, but she acted normal, and asked me if I would come back to her house to help her out with something on her laptop (she lives a few km away from me in another neighborhood, about 20 minutes on foot). I duly agreed, fumbling over my words, and hid my phone as quick as I could.
I was really nervous - shaking, I had been caught, how the hell was I even going to look at her again? I was just trailing behind now, my heart in my mouth, I had a gut wrenching feeling that she was going to tell my parents or something, but then as she was walking ahead of me she just grabbed her ass with both hands - one on either cheek - lifted them slightly, and let go, letting her cheeks just drop and jiggle right in front of me. I was speechless, I didn't say anything, I just quickly took out my phone and pretended to browse, as if I didn't see what just happened. We were nearing my sister's house and I could feel a boner coming on, so I just held it and hoped she wouldn't notice.
Anyway, we got into her house and she told me to wait in the living room and she would go find her laptop. So I sat down and waited whilst clutching my boner from the insides of my pockets. She came back downstairs and into the living room, and got onto the couch opposite where I was sitting to look behind it for the laptop (doggy position, ass right at eye level with me, g string in sight) - it was all getting too much for me, I shifted position and she turned around, she looked at me and then looked down at my lap, and giggled. At this point I was freaking out, I didn't know what to say, but before I could she came over and sat beside me (laptop in hand) and turned it on. As we waited for it to load up, she looked at my lap again and out of no where just put her hand on my cock.
I sat stone silent, and she asked me "what's this"?, before I answered she began moving her hand around my cock, and pulled my trousers down a bit, my cock sprang right up, and she took it in her hand. She began stroking it, up and down, dry. I still hadn't uttered a word at this point, but I wasn't about to get her to stop. She began sucking then, and took her tits out from her bra and top and then took one of my hands and placed it on one of them, I just used my other hand to grab the other and began to squeeze them slightly. She took my cock out of her mouth after about 2-3 minutes and took off her top and bra, got on her knees and pulled my trousers down to my ankles, got between my legs and resumed sucking for a while, my hands were still on her tits but I began getting a little bit more confident here, so I bent over slightly and pulled down her leggings a bit and pulled her black g string up with my thumb and let it slap back down on her back.
So after a few minutes more of sucking and stroking my cock, me legs began to tense up, I was gonna cum, and I guess my sister realised this, as she started wanking me quite fast. She was moaning a bit, the usual stuff, and then she got a bit serious - she was looking right at me with saliva running down her chin, and she began demanding I came all over her face and cover her tits in cum. I could only take so much before it happened, ropes of think cum burst out and covered her chest and her tits. She let out a massive sigh of relief just as I did, and I slumped back into the chair, she stayed on her knees playing with the cum on her chest, before sucking my cock's head clean. She stood up, (I still hadn't spoken a word), and left.