Husband's reaction is totally understandable. I think he could and should be more forgiving, but if he doesn't want to be no one can make him. People have their lines. The mother willing to cast aside her daughter for him is a little off putting but also understandable. I think she should be more forgiving of her daughter since it sounds like the daughter has expressed true remorse and did seem to try to fix her huge error, even if it was too late. I think casting her daughter out will be something she'll regret later. People who make mistakes like this need guidance and second chances, but if you take that away, you take away their ability to become better. That daughter is going to go through a rougher life than what she knew and might not be prepared for it.
I'd say the cousin and extended family bear the most blame out of everyone in this situation. You can't take a small piece of information about people you know and assume it's the complete picture, or that it's a small piece of a situation that's worse than you imagine. This is no different than when people used to accuse others of being witches. It's primitive.
"There is little anyone can do with fanatics. Reasoning with them is a fool's errand. Avoiding them is mandatory." ~Former Conservative Talk Show Host