Sister-in-law told me my wife cheated while on vacation

The players in this drama. My wife who for the purpose of this post shall be called Anne (female 47). My sister-in-law (female 40) who we shall call Shannon. SIL's cheating partner (male mid 40s) known hereafter as Tony. And myself (male, so close to 50 that I can reach out and slap it). We shall refer to me as "me". Usual disclaimers of cell phone and English is my first language, I just suck at it. So my wife had to travel to her step father's house. He is in very poor health, and she went there to help set up home heath. She was there for a week, and we were in constant contact. Her sister is a drunk and a drug addict. At several points during the visit, we were on video chat when "Shannon" came into the room where my wife and her father were. She was buck ass naked, raging drunk. In front of her father. I was mortified, and I'm sure he was too. Now "Shannon" is married, but separated. She has a live in boyfriend. "Tony" is my father-in-law's primary care giver. For the flight home, "Anne" missed her flight. "Tony" was driving her to the airport. There was an unusual amount of road construction, and they arrived late. She had to take a different flight. Not a big deal. After she was home for a few days, the following text exchange happened between me and Shannon. Shannon: they ****ed. Me: who, and did you film it? We could make a fortune on pornhub! Shannon: I saw him leaving her room and smiling. You know she didn't miss her flight! He ain't denying it. Me: well, if they did, he's the luckiest man alive.
Now guys, I know that I have a better chance of creating a fart powered personal jet than this story has of being true. It's just not in her nature. That said, damn that woman to the depths of Detroit's South side for putting the idea in my head.
So, the question: how do I deal with a crazy drunk 80lb woman from 1000 miles away. I can't block her, because the rest of her family has. If something happens to the father-in-law while Tony is at work, I'm the only one she can contact.
Tldr: drunk SIL claims wife cheated. She didn't. I have to decide how to deal with her.


Source.