I have so many thoughts on this.

I did not go to my D22' s grad. and eventho it made me sad, i do not regret it.
This May coming, it will be Son' s grad. He is allowed to invite 5 peoples. He chose: his 3 sisters, me and his father. Will ex-h bring OW? One thing i know for sure is Son would be very upset if he did. He can' t stand OW and he vocalise it often. If ex-h does, which ticket who's ticket will she take? Mine again?

My feelings toward those situations are parallel to yours.

Ex-h walked out without a care in the world for the situation he put us in. The kids were young and because i took on ALL responsabilities, they do not care if ex-h had OW in his life. They still had everything they needed at home.
The only resentment they have toward him is the lack of time he spend with them so now, they, like him, will give him their time when they feel like it. ( in ex-h' s mind, this is all caused by me; not his choices and actions)

His OW will NEVER be part of my family. Unfortunately, she is link to my kids by the hand of their father regardless if they like her or not. If ex-h was to pass away, i know OW would eventually vanish from my kids life.

Now, a thought from my side... when i dated Kevin way back, i had tell him it might take a long time for my children to let him in and it might be my own fault because from Bomb, i kept telling my children not to get attached to any OW ex-h would get involved with as they would not stay.

I know they accept OW only by respect for their dad. They want nothing to do with her side of the family nor her own kids.

I had a discussion with D18 last summer about my reactions ( knots in stomach, anger, fear and where it might be rooted from as i beleive once i find the roots, i can heal). I mentioned my strong family values and been unable to enter a new relationship. She said: grand-maman re-married.. my instant response was: "my dad had passed away 2 years prior.
Grand-maman would not have entered another relationship even if they would have been separated. ( my mother was VERY religious and she raised us with religious morals and values ).

My wish is that my kids don' t think it is ok to do to others what ex-h has done. I know they know. Just like i knew my father' s alcoholism was no excuse for his actions. He was still my father and ex-h is still their father.

Doing the right thing is not always obvious nor easy.

No matter what you decide to do, comminucation with your children is a must. The situation through your eyes and theirs is different. She is their mother but she is your ex. She is part of their life but she made OM the new you in her life. This is something kids do not think about but understand.. would they want their ex at every family events?
Would they fake happy to see them? Or would they stay away from them?
If you chose to go, your son will understand if you keep your distance from her.. no fake !!