My (30m) wife (35f) found out our SIL (37f) is cheating on my wife’s brother (41m) with a guy from crossfit. We’re all on holiday together with the kids for another day and a half, what the fuck do we do now?

Tl;dr, in the title, basically.

Not my relationship, but its a fucking mess, as you can imagine.

Basically, my wife went for a jog with her and another friend yesterday morning, and she was sitting in the back, SIL pulled out her phone, and my wife was able to briefly see the recipient, some romantic emojis, and sweet nothings.

Over the last year or so, she's kind of been distancing herself from us, we think anyways. She always "goes for drives" or "hangs out" after crossfit.

It all adds up in retrospect, but man does it suck. We both have 2 kids each, and her brother makes more money than her, and I think he owns their house.

We really have no idea what to do, they're the majority of our social life, but obviously that's going to change.

Worst part? We're on holidays in cabins directly next to them.

Any experience or advice is appreciated.