If something is going on you will eventually find out one way or another. Contacting your sister-in-law raised a red flag for sure. Does he spend a lot of time on the phone or computer at night? Does he work late or weekends. If he was planning the trip with someone else maybe you calling him out scared him straight and that would be the end of it if there is something going on. Protect your self in case. Keep a close tab on finances. Don’t get caught off guard. What would you want to do about it if he is cheating. Would there be a second chance,, offer marriage counseling. I think coming from a place of conviction and strength is always best. Such as you are mistaken if you think I will stand for this.Never threaten and not follow through. So think about what
you are willing to do in case. I would hope this is all just innocent.