Girlfriend's older sister wants me

Hey guys I’ve been with my girl friend for 2 years, I love her heaps an Im always at her house haning out with her an her family an that. She has an older sister thats an absolute stunner with a rockin body an great tits but I’ve always loved my mrs an nevr done anything that would hurt her. Anyways her sister added me on facey and started chatting to me an stuff, the convo got pretty heated an she actually said “well my backs killing me I would love a massage” she obviously wanted me to go over and fuck her thats what would of happend I guarantee it but I never did it coz I love my girl even though I would dream of fuckin her sister an would love to just go do it!! I never told my girl about it coz I didn;t wana cause trouble with her family an that!
Any who yesterday I found out that my mrs was texting my best mate an that they wen’t an got food together behind my back!! Im not sure if they hooked up or anything but Im so fukin angry at my dog mate and my girl but they dont know that i found out and I havn’t told her i know yet! Any way I cant be sure if she cheated on me or not but she still fukin went an caught up with my mate behind my bak!!! Now I think im gona fuck her sister to get back at her and break it off with her and then tell her i know about her an my mate an that I got fuked her sister!