Just found out my girlfriend cheated

I just found out my now ex girlfriend texted her brother in law that she wanted to sleep with his brother and how should she go about it. What’s even crazier is he texted her encouraging it and saying text my brother you want him to slut you out and I’m sure he is down for that. Mind you we have been dating for 6 years with one break. They have a weird relationship claiming to be “best friend’s” which I already had expressed my concerns about multiple times and she would just say get over it. Of course she texted the brother, and the rest is up for imagination. I just can’t believe I found this all out, I just had this weird feeling to check her messages when I randomly woke up at 3am. I’ll never forget the feeling of insane emotions flooding as I read those text and she’s sleeping next to me. I couldn’t believe what I was reading, yes things aren’t perfect but I never expected her to cheat. She always said from day one of our relationship she had been cheated on by her last partner and it was a terrible feeling and she shouldn’t do that to anyone. That she would want me to just break up with her and never cheat. Well ain’t that crazy…..look who was the one that cheated. What makes it worse is she denied it, stated she didn’t “cheat”, nothing ever happened, I was just joking and never wanted to act on my feelings towards him. She even blamed her therapist for giving her advice to “deal with her feelings towards him”. I’m doing as well as you would expect, it’s hard to deal with this when you were talking about having a wedding soon, etc. I’m upset that she wouldn’t even admit her wrong doing to me, what lies is she telling her friends and family? I know I can’t control that, but it pisses me off that they won’t know the truth about what she did and her brother in law being an absolute weirdo.