It seems to me, that first thing to do is invite your son and DIL to visit on a weekend, for once. And spend some time just with them (not your other sons) for a couple of hours. Talk to her about her job, not in a judgemental way, but out of genuine interest. Don’t make it too heavy, I don’t know how old you are, but maybe along the lines of “when I started work, we barely had computers, and no internet, let alone being able to work remotely. When I saw all the stories about people being able to work from home, through the pandemic, I was amazed” … “I’m really impressed that this is possible” …. “I’m not sure that ‘I’ would have the discipline to do it, when there are other things going on” etc. ASK her what she does? BE interested. As you say “she IS family”.
With regards to your other son, and his marriage, maybe he cheated, maybe he didn’t (I’ve known both to happen). In many ways that doesn’t matter, the marriage is long over, and his ex has nothing to do with the family now. What DOES matter is how this affects your PRESENT DIL. If his ex wife is her friend, then she no doubt believes the story, then she will be very wary of a close relationship between her husband and his “cheating” (and obviously inferred lying brother). The only thing that will heal that is time. And the acceptance that even someone who once cheated on their wife, can still be a great brother, friend, and even a great partner to another woman. There really is nothing that YOU can do to accelerate that process, and protesting your sons innocence will not help. She will see it as you refusing to see any fault in your son, and will fear that you would also blank any suggestion that her husband isn’t a paragon of virtue, and take his side in any disagreement. You love your sons unconditionally, but it doesn’t mean that you need to be blind to their faults.
Tell her stories about them as kids, as examples of natural sibling rivalry. When they argue, recount stories such as “Not again, 🙄, I remember when Johnny did …. And Billy got revenge by …. I never thought they’d speak again, but it blew over in two days”. I don’t know whether she has any siblings, but individual sibling relationships can be hard to understand by others. I have friends, and I don’t really understand their relationships, and I know that others sometimes look at me and my siblings, or look at my own kids relationships with each other and don’t understand them.
Don’t go overboard, you know, like don’t just hit her with all of the above the first time you meet up on a weekend 😂 just build on a relationship with her. Get to know her better. Built trust. And accept that it’ll take time.