Your relationship with him is your business. His relationship with his partner really isn’t.
You can wring your hands all you want, and call it caring. Truth is, you never know the dynamic in any marriage, even if you birthed one of them. You certainly won’t know enough to judge.
If you are having trouble with ‘them’ then own it, and get therapy. For all you know, your worries stem from a ‘you’ thing. Projecting what you wished for your own relationships on him? Guilt about something in his upbringing? Lack of boundaries or jealousy on your part? Who knows.
For the rest, you just stick with the Prime Directive from every parenting guidebook, don’t you? Be there for him, should he need an ear or ask for advice. Fill your own life, over which you have full agency, with the behaviour you’d like to model. Your power and influence lies there.
And as a PP said, be quite careful what disapproval you show on the outside. However justified you feel, whatever evidence you think you have. Your DIL will hold a lot of the cards if/when kids come.
He chose her, you’re stuck with it unless he decides he made a mistake. You can’t choose your relatives, hey?