TWolf that's part of it yes.. and you will find genetic patterns happen... if the spouse is having an affair its more likley that their gender parent did the same thing... in your case your mother in law may have strayed or considred it or was at least wayward at some point.
your actions now will be with you, your son, and your wife twenty YEARS from now. You can look at your son in twenty years and tell him
"I fought hard for your mother and you, to keep us all together... I fought like the devil... You meant the world to me and you still do..."
YOu CAN tell him that in 20 years becuase you ARE doing that... we build a history here for ourselves.. its essential that we build a history we can be PROUD of in the years to come.. and your wife's building a very ugly history she will be ashamed of in twenty years... she has a chance to fix that still.. and you are sending that message to her loud and clear that you want this.. that's important and i can assure you she's hearing you.. if you can sense the guilt then its really building up in her now... she was likley hiding it well before but its goign to eat her up over time TWolf it really does.. keep up the fight ...
It's a shame you werne't fighting this whole time, just got some bad advice somewehre, but you are on the right road now.. lets see what comes of it ok?