How am I wrong for being mad at my son's wife for leaving him for his brother?

Talk to Matt. Don't scold him, ask him if he knows what he is doing, and if he prepared to face the consequences of the situation he has put himself in. Tell him that if this is truly a situation he wants, then he needs to be a man and tell his brother. Something tells me that he will break down right there, because he was probably just a young guy, seduced by an older woman who had ual needs. Now that this is all out in the open, I think Alison will eventually have some regrets and may cut off the relationship herself. It is up to Alison and Lawrence whether or not they want to fix their marriage. Your only role in this is to be a supporter of ALL of them, consoling, and giving advice without ping judgment or showing anger. They are all adults now. The most important one in all of this is your granddaughter. Keep reminding her that everything is going to be okay, and do as much as you are able to do as far as helping with her needs.