"An error doesn't become a mistake until you refuse to correct it." by JF Kennedy
Kijana- It takes a heart to admit an error, and it a takes man/womanhood to even take an extra step correcting it. While most of us point fingers we have either sinned by our thoughts kwa hao hao mashemeji zetu. Ni common sana wanaume kutongoza ndugu za wake zao- but its uncommon for the husbands to admit as you did.
The sin will haunt your life forever thus you need to denounce this ASAP. Imagine in years to come every time u meet her- she reminds you of the sin. And if by mistake u are involved with a person she knows of, you will be insecure of the chances that she will disclose your weakness. From now Don't show any further signs that you need any discussion whatsoever with your sister in law. After all, she (and so are) is young, inexperienced and naive. U need an elder person to help you in this turmoil
two: Confess to God of your sin: make a rational decision of NEVER to repeat it.
Three: in future even if you remarry and find your spouse problematic: Never discuss the weakness with the younger ones. you never know when the two of you reconciles and if your spouse finds that you were bussy disclosing her/him , this will narrow the chances of re-conciliation , that's why in such cases advice from a mature , experienced or religious person is crucial.
four: I have no idea why you are considering divorce but look at your self as well, you may be the cause of your wife's problems. Divorce should be the last,very last option in life. In the first years of marriage couples have great to learn from each others personalities and adjustments are necessary. Please reconsider a discussion with your wife and not divorce.