Ok , and to be fair, I didnt exactly answer your exact questions

1) is this a common occurance? Absoluty and way more often than you can possibly imagine. I am speaking from personal experience, and through professional experience.

2) what should I do? Tough question. The best thing to do, is learn to forgive your sister, but make it known how the behavior is something that has both hurt you, and your mother alike, the later on a level that neither of you could possibly fathom, without being a mother yourself. Console your mother and confirm to, despite what may happen with her relationship with unfaithful spouse (use your own words, you dont have to be so direct and blunt as I , unfortunately not everyone has the strength for that..) , she will always have something more important and unbreakable, a relationship with her daughter that loves her unconditionally. Together you both can learn to forgive and move on. I cannot tell you what you should tell your mother In regards to him, as sharing statistics, red flags, and science has already upset you dearly. However, what I would say is that , despite what feelings may persist, that feeling of distrust will never subside, and together you can learn to even evolve, and learn to forgive his actions, without being fooled that it wont happen again, with possibly someone else. There are plenty of loving and loyal men out there (although, as history shows, are rarely appreciated... as the responses dictate), when she is comfortable, she will find someone else. And if not, she always has you, far more important than any man.. and she still has time to repair the relationship with your sister, and move on

Best of luck, truly meant harm. I'll accept the anger as collateral damage. Wish you the best, and pray you have the patience to read through,quite possibly,the most direct words you'll get on this topic.