In- Law!

SO it was April 1 and my sister and I went to her best friend's baby shower we had a blast it had been a while since me and her had spent time out of the house. When we got home me and her husband did our usual ride around and drink. We stopped by a party I was given too much alcohol. When we got home we finished what we had. I was horny so I said I gotta get out of the car (not loooking at him like that but the alcohol was tlking). I got out and he kept pushing the issue as to why I got out. I wouldnt say. We proceeded to my room(normal activity) and started playing and as he tickled me which rarely happened I guess a spark happened Idk. Anyway we were laying in the bed playing with a box cutter (my sister works at walmart and i stole her boxcutters because i was a cutter) we pointed to body parts to cut off (joking) he pointed at my breast I laughed(joking) and then he touched one my head went spinning. Then we kissed. I was like damn what's happening. So we stayed up all night messing around(drunk) the next day came my sister and her grandma left us home alone and thats when we had sex. After that we just started having sex almost everyday. Then we got closer than we already were. I made the confession one night he was holding me that I loved him he siad he had been feeling the same way. We had an accident together and somehow that rose supiscion my dad told my sis (we have the same dad) i was sleeping with her husband she gave me a deadline to get out. I moved across town he helped me. We were serious before that but I was living with them so it was easier for me to continue what I was doing with him. He said once i moved we would break up but we didn't. It was a year in after i moved and things were crazy i loved him. i would cook for him and just do all of the things that she was suppose to do for him awhile i hated him and loved him for making me feel the way i felt. he would threaten to leave her but i knew he wouldnt and because she was my sis i wouldnt let him(not because of me). I got pregnant I knew it wasnt his but i couldnt tell him and then i lost my baby. it killed me and him and to this day he says he would be the happiest man alive if i had his child. He and I broke up bout a month or so ago but thats when things got crazy right before the break up. We went to his job's xmas party and he was saying i was his wife it made me feel great to be on his arm and he make that claim. that same night he had too much to drink and couldnt drive home(which is a night he goes home because she dont work) it was about 3 when we woke up and she was outside calling both of our phones she said she knew he was there and he needed to come out. that night a lot was revealed to me. he had made our affair seem fake she didnt believe it she only balmed me for trying to seduce her husband and said i was the only one at fault and she knew that he was at my house everyday. he spent the night when she went to work we went out to eat and to movies as a couple but she still blamed me he had me driving 'his' car that was hers and anyway off of that but that fri they "called it quits" he stayed with me all day sat and sun went home to her i called no answer so i decided to go ahead and head back home to mom. I packed my things and they helped me move out of town and although we ended it he still comes here and its like things are back to "us" again IDK what to do you guys I need help. Iwas wrong and am worng for being with a married man and especially one that belongs to my siter. I love that man with everything in me but that's my sister husband they were together 11 yrs before they married and have been married for 3. me and have been in an affair 1 year and 8 months and i dont know how to stop. I'm 19 he's 33, he's settled and i'm still finding my purpose, i have to finish college and he's straight with his life, he loves her and me I can't be the other woman much longer!!!!!!! Please help!