I'm sorry that you are hurting today. All I can say is that she doesn't have it that great. He is a liar and a cheater and if he wasn't loyal to you then I doubt he will be loyal to her. She probably doesn't care if he's faithful to her or not. She began a relationship with a married man. She isn't living your life, you probably wouldn't do that to someone else and be ok with it let alone flaunt it.
Are you planning any vacation time for you and your family in the near future? Can you go out and get a massage, manicure, hair, clothes, makeup and go out with girlfriends and bitch about them while they are on this trip? Can you plan an extravagant trip with your children/grandkids for you to remember? She doesn't get the love they have for you. That's for you. She isn't entitled to any of that.
Its so good to have this space to vent when having a bad day!
Maybe you can make a plan for a future trip, start planning it, even start putting away spare change or a few dollars a week toward it. That way you can shift the focus to your awesome trip whenever they are flaunting their crap.
It's all about detaching, and it takes time but it also takes discipline to change your thinking. Sure, you'll have days when you slip back a bit, but it will get easier to get past it each time. Focus on YOU.