"i' h8 my sister, but i still cant stop thinkin bout him"

I use to be in the most fantastic fairtytale like relationship where i couldn ask for more. i gave it up because he got so jealouse cuz i was so pretty. i gave up to easily and called it quits. Four years and 2 kids later i ended up with his BFF, who was almost just as preciouse as he was but not at all jealous, but turned out to be too much to handle. There was not a day in those 4 years that i didnt think about regreting everytning with my ex. I explained to my sister for 2 of those years about what a gr8 deal he meant to me and that i was going to get him back. She exclaimed she was gonna help. next thing i know, she's spending odd amounts of time at his home. I asked them both, seperate times, if they had anything going on, n both denied. But one day i get a call from my sister saying she was gonna come clean n that she was seeing him for the past 2yrs. But she didnt know about that eigther because she recently found a letter from him to me that he couldnt live with out me. Laughing at that, but heartbroken i couldnt believe it. I dont think i would be able to forgive her for lying for the past years. What do i do. His image and touch and kiss wont leave my mind alone, but he's also in the arms on MY SISTER who i trusted. its driving me to insanity. i know he still cares about me but i dont know if he is tryin to get back at me eigther for going out with his BFF. We cuddled couple of timewithin those 2 years that they were QUOTE going out, so wut does that mean? i can see it that he still cares but to see my sister (the person i have to live with 4 the rest of my life) kiss and hold the man i agony for. i feel like im to blame for ever having this WAIT thing gone too far. Is it normal to think so much of someone from your past and then hate your sister for the rest of your life because she took away your everything?