He cheated. Should the wedding still happen?

My husband found out last week from friend that his sister's fiancé had been changed on her for a year. My DH even met up with the other woman after he found out who she was (they used to work together), and this poor woman has no idea she was in a relationship with a man who was already involved because he spent so much time with her, even sleeping at her place most weekends and going away with her in vacations. Well, long story short, last Friday, my DH told his sister about the affair and showed her the proof he got from the other woman. His sister confronted her fiancé, called off the wedding, and even made him confess to her parents about what he had been doing. The next day, however, she said the wedding was back on. She's getting married this Friday, and I don't know how to support this. I know is her decision to get married to a known cheater, but how can family just smile and say congratulations?