I'll agree with the idea that to a degree, she has to stick with the plan. Too much pride, now. (One of the 7 deadly sins). Letter to sister is just another circumstance that you must not allow to sway you.
Here's your job for the next ??? months. You are to love your wife as you promised you would do. Not because of what she does for you - but in spite of what she does TO you. That's the love she needs to feel.
Me - 43 and She -36. No kids. Married 7 yrs - Together 14 yrs