I keep running through my head that had I just left my sister in the dark, I would not be facing divorce and would have a much better chance at turning my marriage around.
2x4. Sorry.
Let me reword this for you: "If only I had allowed my wife to quietly continue her affair, if only I'd allowed my wife and brother in law to keep my sister in the dark, I'd have a better chance of turning my marriage around."
Does that sound like a rational place to be?
We ALL have the "if only I'd done this" thoughts. I promise you that before this is over, your W will absolutely, positively, put the blame on you for her affair, for continuing her affair, and for deciding your M was over because YOU had a problem with her affair.
Easier said than done, but you HAVE to start focusing on you. HAVE to work on being the best Tate that you can be. It MIGHT save your M, it might not, but it absolutely WILL save YOU.
You have no control over her. Over what she does. Who she does it with. You only control yourself. Latch on to that, and BE the person YOU want to be. If you want to fix your M, then be the H only a FOOL would leave. Even if you lose your M, at least you've still done the work to be that man.