How do I bring this up to my fiancé?

Heads up… This is me venting… So my fiancé just received an email from a woman saying she needs to talk to him. We share a tablet, and both receive email notifications on it. He is currently at work. Yes I was nosy, and further investigated to find out who this woman is. It turned out to be his ex wife’s sister. His ex wife has been trying to get in contact with him, but he changed his number. It’s a longgggg story, and I can go into more details in the comments. Anywho, they supposedly divorced because she cheated on him and had a severe drug addiction. From what I’ve observed she appears to be very toxic and manipulative. She would always beg my fiancé for money that has nothing to do with their children. Which is why he is not currently speaking with her. The children live with his parents. The last time they spoke, he advised they speak through a co parenting app moving forward. So that there conversations would strictly be about the children, and she stop asking him for money. She completely got mad and cursed him and me out. She then insinuated that he touched her firstborn daughter. I felt disgusted and questioned him if it were true. He denied it, and after long thought I believed him. What she insinuated just didn’t add up.
Well today, when I went to the sister page… She had a link that led to her online diary entries. She is also a struggling drug addict and bipolar along with many other disorders. Apparently mostly all of their siblings are fu*cked up. However, on one the diary entries she wrote about how her sister, “my fiancés ex wife,” is a terrible mother. She also wrote that the step father, “at the time” “my current fiancé” touched the daughter and the mother swept it under the rug.

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Sorry I can only type so much on here, but I have so much more to say! I feel sick to my core right now. I don’t know what to believe. As her and the sister are clearly drug addicts with mental health issues. I however take sexual abuse VERY seriously. As I was raped by my own father as a child. I feel so afraid and lost. I don’t know what to do, or how to bring this up to my fiancé when he gets home. What should I do?
How do I bring this up to my fiancé?
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