Can’t Stop With Girlfriends Sister

I seriously can’t stop cheating on my girlfriend with her sister. It’s been 3 years since we started dating. She’s now 25 and her sister is 19 now. Her sister always flirted with me and about 8 months in, I gave into lust. We both keep saying we need to stop, then the next family gathering happens and she’s either riding me somewhere secret or I have her bent over something away from everyone. It’s so wrong and I feel guilty because I want things to work with my girlfriend as I came to really care about her. Yet I can’t stop being unfaithful. We have fucked on her bed, my girlfriend’s bed, their bathroom, my car, their backyard, and their garage, probably a total of a dozen times. I’ve honestly lost count. It’s just too amazing to stop. I confess this now because we just fucked like rabbits while my girlfriend went to the store to get stuff to cook. It was just us so we tore into each other I came so much inside her. She knows what she’s doing too because she always says dirty things like “fuck your girls little sister” or anything she thinks of along those lines. I hate myself for this and yet I can’t get enough