Had sex with my bestfriend sister (need advice) SeXStoRY

Hello I just open an account here to get some advice on my situation. I don't know if this the right place. anyway here is some information about myself and what happened, ok I am 18 years old from canada, Toronto. well i am originally from Egypt, and moved here 5 years ago, and since moved here i been friend with Ian, well that is not his real name, we been besfriends for almost 5 years now, he help me learn English and the Canadian culture, anyway last Monday ended up having sex with his older s****r, and i am freaking out right now don't know what to do, i need any advice, any help. ok me and ian are 18, his s****r lets call her Ashley is 23. i known her since i been friends with Ian and i am always over at there place, and i always had a small crash on her but nothing too big, anyway i will get to the point i don't want to write too much, so i am just going tell the basic no details for now. cuz i need help today if possible.ok so i was there last sunday, sl**ping over there and in the morning was just me her and ian, and ian left the house to go to the gym, and i swear she is the one started the whole thing, i was trying to say no, but you no it was hard to say no, she was basically bagging me to fuck her, we ended having sex on Monday, Tuesday and Wednesday, the thing i only been with 2 girls b4 her, but the sex with ashley was the best by far, she was a freak, and the thing she has a bf, a big guy, which scary me if he found out, but i don't care about that, i hate the fucker, the best thing about having sex with her, is to rub it in his face, caz he such an asshole, with even me and ian. anyway now i feel guilty and don't know what to do, i mean i wanna stop but it is so hard, i never had a girl like this b4 telling me to make her my slut, i mean i don't think i am a good looking guy or any thing like that, so chances like that does not come too often for a guy like me, plus she is hot, and i really don't give a shit about her, she is a slut, just feel for ian that is my only thing, don't know what to do about that, the guy only lives with his s****r and mom, and his mom kind of known for sl**ping around and he hates her, i mean i thought i would have a better chance with mom if anything, but with his s****r never thought it would be possible,just happen so fast, and i even have the text message to prove she is the one after me, i just got a text form her last night 4:21 am saying wanna come over, i'll do things no other girls will do, i mean what i suppose to say to that, and she keep telling i am the best she ever had, which is i don't know if it is true, i only been with 2 girls b4 her, and not that experience and no one ever told me that, but i think i do alright, anyway i was asl**p so i didn't respond to her text, and about an hour ago got another text saying r u there, i have not responded yet, not sure what to do, i love the sex, she is basically my personally slut not even joking, i known her for 5 years and all the sudden she is all over me, and i love it, but same time i feel so guilty, any advice will be helpful and if anyone has any questions plz ask, i think there is privet chatting her too, so wherever is fine, it is 8 am here and i am going there around 8 pm tonight, so any advice