@Tiredoftattler I'm going to suggest that you are an outsider to this situation and not a neutral outsider at that. It is very clear (and probably more so in real life) that you very much support and admire your aunt, and are determined to see her as a brilliant example of a reasonable human. That will not be lost on your cousin or his wife (and ex).
Even the way you describe everyone as appearing to be happy with the situation makes it clear that you are merely observing from a distance. If everyone is as rational as you claim, then obviously they aren't going to play out the family drama in front of their extended family and the world at large. Why do you assume that you would know how your cousins wife feels about any of this? Or what kinds of discussions and compromises there are within her marriage because of your aunt's unwillingness to build a relationship with her son's wife and the mother of his youngest child?
From the details you have given, there are big holes in your rose tinted story.
Families are full of relationships that are not chosen. We don't choose our parents or children. We certainly don't choose their spouses or friends or coworkers. Pretty much all of us do our best to build at least cordial relations with our in-laws, unless they behave in a manner that is truly unacceptable. Even then, you will find us politely putting up with hideous family members at family events, and not making them all about our internal family dramas.
From the account you have given, your aunt has clearly never made any attempt to build a relationship with her DIL. The extended family (you) know that she blames her for the break down of her son's first marriage and has refused to build a relationship with her on that basis. Yes, I'm sure the DIL probably doesn't feel that spending time with the woman would enhance her life. But she's had a hostile MIL from the outset.
Given that your aunt has failed to build a relationship with her first DIL. It looks like a bit of a pattern in which she is incapable of accepting that, as the mother of an adult child, it is not up to her to judge and police his relationship choices. When your kids grow up, you need to accept the choices they make whether you personal like their partners or choices. You do your best to respect their partners and to accept that they are part of your life.
It is not acceptable for any MIL to decide that her grandchildren's mothers are merely vessels that brought them into the world and extraneous to her purposes. What you seem desperate to describe as treating both the mothers 'equally' is very clearly a dismissal of their role and a total failure to recognise the difference between your son's wife - who IS socially and legally a key part of his life - and his ex. It is never a neutral act, especially not when the entire family knows she disapproves of her DIL and blames her for a divorce.
None of the actual details sound like the kind of mature and reasonable 'detachment' that works for everyone you are determined to see it as. Indeed your comparison to and implied criticism of the much less 'cordial' relationships people are describing here are deeply unfair and miss the point. What you are hearing here is the thoughts and feelings and issues that are raised behind closed doors when women are faced with hostile MILs who think it is at all acceptable for them to decide what is and is not 'fair' for grandchildren in a blended family (rather than those children's parents).
It's not at all unreasonable to suggest that behind the public face of your cousin's marriage in your family, there will be strong feelings about his mother's choices, attitudes and behaviours.
My ex's parents are really lovely, welcoming people. I adored having them as in-laws and had a very close relationship with them. In fact, losing that was a big blow in the breakdown of our relationship. We are still friendly, but we all recognise that the primary relationship needs to be between them, their son and (through that) their grandson. He has to be able to move on and obviously they should see his new partner as part of the family in ways I just am not. It would not be fair to my ex, or his partner, for them to invite me to Christmases etc. If he has more children with her, then we will not simply be just mothers of their grandchildren. She would be their DIL and part of the family; I'm not part of the family any more. Recognising that doesn't disrespect me as DS's mother. But failing to recognise it, would disrespect my ex, his partner and their relationship.