This is based purely on my observations, but it seems like infidelity and WAW syndrome move through female social circles like a contagion. In my experience when I have have a female client who just walked out or cheated on her husband, in the vast, vast majority of cased one or more of her best friends, sisters, mother or daughter either just did the same, or is about to. I would say that the odds that your sister in law has had an EA or PA are extremely high.
Where I would apply this to your situation is that the more your wife communicates with her sister about their situations, the more they will likely encourage and reinforce each other to move away from their respective marriages. This is often called "toxic-friends". In my experience as long as a woman has either a close toxic friend or an OM they will continue to distance from their husband no matter what he does. Only when they lose this support/motivation do they seem amenable to working on their marriages. Obviously your wife and her sister will always be sisters, so unfortunately it is unlikely that they will lose each other's support anytime soon.
Granted, I am more of an expert at ending marriages than saving them, but I don't think there is much you can do about your wife's feelings for the time being other than the 180, making yourself generally more attractive, not doing anything to help her separate from you, and just wait things out.
