Marital break up causes wedding dilemma

If she has any sensitivity she will wish them well but say she won’t be attending. Weddings are stressful enough without having friction between two people especially if it’s just a small affair. If the split is quite amicable it probably wouldn’t be a problem but from my experience it takes a long time before a divorced couple can move on and become friends, and sometimes it never happens anyway.

if they've both been invited then they can both go and act like adults surely

I don't agree that blood relatives come before others, there are many people that I like more than my relatives.

In this case, I would agree that it is up to the couple who they want at their wedding, and the other guests should respect their wishes.

This is not on you. DS&his future wife future wedding, their invites.

It's up to the couple to decide - if they love/like both parties and want them both there then Uncle and ex-partner should suck it up and behave like friendly adults on the day at the wedding.
In other words leave your problems at the door on your way in and pick up them on your way out.
There is no reason that the couples day should be spoilt because of a relationship break down.

If the Invitations have been sent out it would be extremely discourteous to withdraw them, so it is up to the two guests concerned whether they attend or not.

As I read the responses, I'm leaning more towards the fact that my Unhappy Couple (not exes yet- split is recent but not going to resolve) should sort this between themselves and not put pressure on my Happy Couple who are already beginning to buckle under the toll of wedding build-up, work and new baby and being who they are, won't want to offend anyone!

It is up to the couple to work on their attendee list, DNA or not.

I admire the now exwife, who knows this is not about her, unlike the uncle, who feels entitled

If they. were only married for 3 years and the divorce was recent I would say she shouldn't be invited. They might still have issues and you don't want the day ruined.
Unless you are 100% sure they are friendly don't invite both.

Blood family really shouldn't be the priority simply for that fact. Many would prioritize friends over distant relatives. A wedding is about the couple and who they want to share the day with

Well obviously it's up to the couple but I would think that the uncle would keep the invite. His ex may well find a new partner and become engaged with a new family but a blood relative will always be just that.

It is up to the couple marrying who they would like to be there

In my opinion: Couples who split up need to do more work on being able to be polite and cordial to each other. Just look at the harm it causes children when parents cannot get on after splitting up