I'm not sure how to handle this.

I was close to my husbands littlest sister. I watched her grow up some of her life. I changed her diapers and stuck up for her while she was still small. But my husband and I got a house about two years ago now. And we told everyone the same thing, no sex in our house. I found out about a month ago she had sex with my best friend at the time at my house. Obviously I'm mad so I yelled at who my best friend was and I was gonna wait to talk to her in person. She avoided me like I was an illness she didn't want. But the more I talked to my best friend about it I found out she was gonna lie to me. Tell me that they had sex somewhere else. And he sent me pictures of their conversation. My best friend said "I thought you told her" and my husbands littlest sister said "no I'm not that stupid" . So I went ahead and her and I finally had a talk about it and she tried to get me to feel bad for her. By saying "I love how you only care where it happened and not that I got my feeling hurt. That I was sad he lead me on. " she ever even let me get my feeling across to her. She also told me that she was gonna tell me about it after she didn't feel so used. But like I said earlier I have a picture of her saying she was gonna lie. I don't know if I should believe her. And I'm not sure if I wanna be friends with her anymore. Does anyone have any advice for me?