This sucks, and I'm sorry your sister is going through this right now.
However, I can say -- having sat with a friend as she cried on my shoulder -- that people don't cheat unless they're already unhappy. My friend cheated on her husband about three years ago -- to add insult to injury, it was with a neighbor who happened to be her ex-boyfriend. Not only was she my friend, her husband was my friend too. They ended up going through a LOT of counseling, and today their marriage is stronger than ever... but when she cheated on him, it was because there was something lacking in the relationship she had at home.
Give the guy a chance to explain his side of the story. Cheating's never okay, but sometimes there's a reason for it other than "can't keep his dick in his pants".
Although, if it turns out that IS the reason, feel free to swipe some hair and fingernail clippings and mail them my way. Muahahaha.