my family is so fucked up

my sis went back n told my bf that I cheated on him only becus I made her mad one day and now I'm having to relive the past everyday becus I hurt my bf soo bad that he just can't get over it and I don't blame him cus he really is in love with me... now its been almost 7 yrs ago since that happened and everytime he get mad at me he always bring that up he just stay stuck in the past.. I hate my sis so much for doing that I have never done that to her eventhough she is way more experienced then I am.. I don't Care how mad she make me I can never tell on her I just don't understand why would she bring misery to my life