Question: If your brother had sex with your wife which caused you and your wife to get divorced over it, and then your brother started a relationship with your wife, and they claim to love each other, do you think you could still go to thanksgiving at your parents house with the family?
Answer: That is tough. I would let my parents know that I am still upset about the situation, so I could not guarantee that it is going to be an enjoyable Thanksgiving for anyone if they show up. Seriously, I would either sit this one out or insist that they not attend. It is a joke to think that everyone sits there like it is a big happy family after your brother and ex-wife stabbed you in the back.
The best way to get them back will be to move on and find someone better. That is a situation that will undoubtedly go bad. It started on something bad so it has no chance. You just need to be there to smile in their face when it happens and be happily in a new relationship so that you do not look back. Karma will catch up with the both of them. As for Thanksgiving, might I recommend some sliced turkey from the deli section of your local grocer? Not worth sitting through that meal.