Need advice

Back in the beginning of October my fiancés ex-wife was here visiting with their daughter (We'll call him A and her C). Fast forward to the end of October when we announced our engagement on Facebook, I get a message from the C's best friend saying that A and C slept together while she was visiting. A denied it, his brother says they were never even alone together. Once I calmed down and got some sleep, I realized that I believed him. He loves me. He does not and has not had feelings or physical attraction to her for a very long time, but I've always had my insecurities and felt like I'm not good enough for him, so in the moment, it felt like my worst fear had come true. A was out of town working when I got this message, and about a week later I asked the guy he was rooming with how he was acting when I called to ask about it all. He said A was shaking and could barely breathe and that once we hung up A called his dad and was practically in tears swearing it didn't happen and asking how to get me to believe him. We both blocked C and her bff on Facebook. A was only communicating with C through his mom. After about a month things had settled down and I told him that he needed to go back to texting her to check in on his daughter. I trusted him and he didn't need a middle man, and all has been good.

This morning C messaged me (I unblocked her so that I could get some pictures of his daughter for Christmas), saying again that she's sorry for sleeping him A when she was here visiting and anything else that she has done that may have hurt our relationship. I didn't reply and I won't because we still don't have set custody arrangements and I don't want to do or say thing that could potentially hurt his chances. I really want to believe that nothing happened between them, but I'm so hurt right now, I don't know what I feel or think. I would really appreciate any insight.

***edited to add:

Her bff messaged me just a couple HOURS after we announced our engagement. That's what really made me feel like it was a lie to try and hurt us.