I (20M) found out my girlfriend (19F) of a year has been having sex with her brother (26M) for the past year. I need help cause this is beyond any normal gf/boyfriend issue.

Throwaway. I need some help. This case is beyond normal advice and needs something else. I'll make it as short as possible. <br><br>I am 20. My gf (19F) and I have been dating for a year. She had a large social media following before we dated. She introduced me to a summer fling of hers from 5 years ago. I'll call him Cas. Cas is her brother's best friend from childhood. We all became close friends. I trust Cas 100%. Not that that matters, but we were all in the same class rank. So I'm 20, gf 19, Cas 26, Cas' brother 19 (gf's age), gf's brother 26 (Cas' age). <br><br>I'll cut straight to it. Cas called me up to his house tonight. He told me he needed to tell me something about my gf and her brother. He said that for the past year before we dated they were having sex. He showed me evidence. Their texts, 2 videos of them having sex, and pictures of them cuddling in bed. He told me that he had been watching from afar for the past year. He said that he's never forgotten about the brother, and that he did it in between himself and my gf.<br><br>I feel sick. I took pictures of the evidence, and I may or may not show it to her. I honestly don't know what to do. We had plans to move in together in 2 months. I could see her tears forming up when Cas told me and showed me. I honestly don't know what to do.<br><br>I'm lost. I just need to talk to someone. I don't know what to do. Please send me your advice or just a message. I will update this post when I talk to her.