Now he's my step brother

First before i tell my story i would like to discribe myself. I am 18 years old. I am a brunette (long hair) I have a great body and alot of men seem to like me. This all happened 2 years ago when I was 16 and he was 17
Ok so anyways.
I had this really big crush on Dean who is 19 but i couldn't go out with him because his mother and my father were getting married so i always had to keep my emotions locked up.
My father soon married Deans mother and we all moved into this huge house over looking the beach. Dean and I always flirted with each other. (I am a born flirter).
Well one week my father and Deans mother went away on a trip to New York City. So Dean and I were left alone. (This happened in may when it was the begining of are summer vacation away from college)
Well for the first couple of days Dean and I just sat around watching movies and just goofing around.
So it on Friday night I came home from a friends house and saw dean on the couch with some blonde bimbo and they were having sex. I got all horny inside because he has like a huge dick. I wanted to be that blonde girl so i could ride him. But i left them alone and snuck back out the door before anyone could see me.
On sunday night i was suppost to go to my best friends families cocktail party so i bought this really cute black dress that showed "Some" of my chest. (Wanted to look sexy).
Well i didnt wear a bra with the dress and i wore a white lacey thong.
So i walked down stairs and i noticed i had 20 more minutes until i really had to leave so i went to go sit on the couch with Dean. I sat down right next to him and he turned off the T.V and just staired at me.
He said "Damn your looking sexy tonight. Where you going?"
So i told him that i was going to my friends house and then he just kept staring at my breast so i pushed him and told him to stop. He then did somthing that just changed everything. He grabbed my head and kissed me. We started making out and I was getting hotter and hotter. He unlaced my dress and started kissing my whole body. I then took off all of his clothes and started giving him a blow job. After that he took off my thong and we started humping. I could tell he was really into it because he was moaning and sweating. (This wasnt the first time that I had sex but it was the best). Well after about 15 minutes of the same postion i decided i wanted to do it a little differently. I got up and leaned over the couch and he got behind me and started fucking me like crazy. It felt so good i just could stand it any more. I later started sucking his dick again and he cummed all over me and that was the end of it.
Dean and I now secretly sleep together when my parents are gone or are just to busy to notice. We are both seeing other people and i am sleeping with my boyfriend Richard too but he doesnt give it to me like Dean does.