Clearly there were red flags before you two got married. Why would she sleep with a guy, right before or after you got married? That was never really discussed.
What can we learn from this broken relationship that was doomed from the start?
1. Get to know the person better than any other even if it takes 2-5 years of dating.
2. Find out their passions and what makes them truly happy and find ways to make them happy often.
3. If something is wrong in the relationship find a way to resolve it without trying to hurt the person you love.
4. Every day find something nice to say
5. Build trust in the relationship/marriage
6. Be supportive of each other.
7. If you know the issues with her sister, don't use it as ammo to launch in her direction.
8. Always remember why you married this person. What makes them so special that they deserve your love.
I hope everyone someday finds a person to love and who will love them back just as much!