Getting to like my step daughter

My wife and I have been together for fifteen years now and as you might expect things have become very run of the mill between us. The way we are has never been helped by the fact that when we met she had a two year old daughter who was a bit of a brat and got worse with age. This girl is called Sarah. I can honestly say that despite Sarah being a very fit and attractive girl I have never had any remote thoughts of anything sexual about her until just recently. Sarah is now 17 years old.
My wife and I both work shifts and so share all the chores in the house. My step-daughter does absolutely nothing for work or around the house. My wife lets her get away with everything and I have never bothered to challenge it but inside it really pisses me off.
About a week ago I was putting a load of laundry on for washing when I noticed a pair of Sarah's panties that had a slight mark on the gusset. For some reason this became a game changer. I put the rest of the washing on but took the panties to my bedroom. I thought I happened to be home alone so I just laid on the bed and took my cock out. It was already hard with expectation. I didn't bother smelling the panties, I just wrapped them around my cock making sure the gusset was pressed against the end of my cock and started stroking away. I hadn't had sex in a while so I shot my load in a very short space of time. Afterwards I cleaned up, put the panties back in the laundry and had a shower.
Within about twenty minutes or so I was in the kitchen drinking a coffee. I took my phone out to check it and I had a Whatsapp message. The message was from my step daughter and it had video content. On playing the video clip I saw a few seconds of me wanking into a pair of panties. The clip was very quick and at a distance but it was obvious what was happening. It seems I hadn't been home alone and now I was having one of those moments where you want the ground to open up and swallow you. There was nothing else with the video clip at all and then after what seemed like an age another message came. This message read 'do you think you could come to my room'.
Ever so slowly I made my way to Sarah's room like the walk of the condemned man. Sarah's door was open and she told me to come in and shut the door. I tried to speak to try to explain but she instantly shut me up and insisted I sit in the chair. Sarah told me that I was to remain seated and say nothing or she would send the video clip to her mother. She said I was to do exactly what she told me to do. She then told me to remove all my clothes and sit back in the chair. I didn't see that I had any choice but to comply. There was nothing to write home about, I can't recall my cock ever looking more limp. Then in disbelief I watched Sarah remove all her clothes except for her panties. She had quite small but really stunning look tits. She certainly had my attention. She laid on her bed and went straight to playing with her pussy through her panties pushing the material into her opening. She was making soft moaning noises and really pulling the panties between her folds. I could see that her panties were getting wet and unsurprisingly I was getting very turned on. Finally Sarah took her panties off and threw them at me. She told me to repeat what I had done earlier adding that it would feel better with wet panties. I had to hold them to my nose and they smelt so sweet. I wrapped my cock in them and started stroking. I was now really enjoying the moment. Sarah asked me if it felt better with the wet panties and I told her it was much better. Sarah then said that she couldn't believe I was sitting there wanking into her panties when she had a cunt that wanted filling.
I climbed onto her bed and went to lick her pussy but she pulled me up to her face and told me to just put the head of my cock inside her. It was like pushing into a vice but I got the head into her and somehow managed to stop at that. Sarah held my face and stared into my eyes. I heard the words 'so step daddy do you want to fuck me'. I replied with the most pathetic sounding 'yes'. Then Sarah said 'I know you've never really liked me because you think I am such a brat but now is your chance to punish me, make me have it, force me to take your cock.
That was it, I rammed my cock full length into her tight cunt and didn't stop. I was sliding in and out with such force. I got her legs up on my shoulders and pinned her going balls deep with each thrust.
Despite how wet she was I could feel her gripping my cock so tightly. Sarah didn't hold back. All I could hear was "fuck me step daddy, fuck me, harder harder, I want to be punished, come on rape your bitch'. I couldn't hold back I said I was going to cum.
Sarah yelled out "don't stop don't pull out dump your load deep in my cunt"
I held still with my cock completely buried in her cunt and pumped every bit of my cum exactly where she wanted it. I wanted to stay like that forever but with my wife due home it wasn't to be. However Sarah did tell me that this wasn't going to be a one hit wonder!!