How can I tell my girlfriend that I have been cheating on her with her sister who is also my sister-in-law bec?

because she is also married to my brother who left his first wife for her and now his first wife married my other brother who is a preacher and he hasn't a clue. I don't want everyone to know. I need some hekp on this. P.S. wee all live in the same trailer park and it's very hard to keep it a secret and if I don't tell her, then someone else might amd that would break her heart if she heard it from a different source. Any help, please.