HELP!! Do I let him know I know??

So my I had to use my sisters phone on Saturday. I saw messages from my husbands cousins husband. I hope that makes sense. She is so lovely me and my other half love them as a family. The messages to my sister were like, I’ve looked through your Facebook, love your tattoos you look like a band girl. I’m in bed all stiff. I’m going to relax in a bubbler bath maybe you should and send me a pic.

My sister only replied. What would your wife think about these messages. She was not leading him at all. He persisted. Would you say anything? I can’t tell his wife she’d be upset but I might message him to let him know I know??