Hi, JustHow - welcome to SI.
I'm so sorry for what you've experienced. You mention it's been years, but it feels like yesterday.
Can I ask how long ago this was discovered and when the divorce took place? The fact that you feel like it was just yesterday suggests to me that you have a lot of unresolved trauma that you've been unable to move past. Trauma from infidelity (and the fallout) is just as real as PTSD that soliders have post-war, and there is no shame at all in it at all. Are you still in therapy? Have you started to build your own life separate from your ex?
Yes, I want them to have a relationship w/him, I have heard from them "he's still my dad". Yet, I have not ever felt supported my son's. They have never held him accountable. They still have their wkends away w/him ect.
It's natural to want justice - there is zero fairness in any of this. From where you are sitting, it probably feels like you are the only one who has any consequences at all and I imagine seeing the OW play "happy family" with your adult children is excruciating. That said, and please know I'm saying this in the most gentle possible way, it is not fair or healthy to expect/ask your sons to be the ones to punish their father for this. I say this as someone who was (roughly) in your sons' position - my dad left my mother to be with the OW when I was 19 (we were not actually told about the OW until it came out a few years later). I was technically an adult at the time, but no one prepares you for that.
The relationship your kids have with your ex is (and should be) separate from the relationship you have with your ex and they need to figure out and navigate that on their own. They can and should be able to have a relationship with their father, without feeling like they are betraying their mother. Your own injury prevents you from being a confidant in any of this but they are hurting too. Whether you see it or not, I imagine that their perception of him (and themselves - kids often identify with their parents to some degree) has been irrevocably altered. If they are anything like me, they will still be unravelling the fallout from this decades later. As I approach 40, I have so much more respect and reverence for how my mother handled things than I ever could have at 20.
The best you can do to support them through this is to respect their space, and not make the time you do spend with them more difficult or complicated (or emotionally loaded) than it has to be. As a married adult who does not live in the same city as either of my parents or inlaws, navigating family time and holidays is already complicated. Having divorced parents who I would not ask to be in the same room outside of an absolute necessity (ie. weddings/graduations) makes it harder. Having 3 different households to visit necessarily results in each receiving fewer visits. Especially when I was just starting out at work and had limited holiday time, I used to exhaust myself trying to see everyone while pleasing no one ("it's a shame you can't stay longer...."). Holidays are supposed to be relaxing and managing all those expectations was anything but. I am grateful that no one scorekeeps anymore (at least not out loud ).
Play the long-game, and trust that your kids will eventually come to their own conclusions about who their father is - because if he's truly a narcissist they will. Focus on your OWN relationship with them and stop trying to compare it to the one they have with your ex and the wife-tress (I know that's hard) - things do not always appear as they seem. Do not pump them for information about their dad and the hw. Do not make sideways comments if their father's name comes up in conversation. Do not use them as a sounding board to vent about the unfairness of it all - that's what a therapist or SI is for! Do not guilt trip them for not being able to spend Thanksgiving with you this year, because they have plans with their father - find another day that fits with their schedule and make the most of it. It'll work itself out eventually.