Sleeponeday - at what point will you consider the OP's DSD to be old enough to take any responsiblity for her actions?
What actions? Opting out from the unbearable pressure of being trapped between two parents, and owing love and loyalty to both? She didn't create this situation, she simply chose the option that caused her least pain.
I'm trying to say this as nicely as possible: just because her actions caused pain does not make them wrong, any more than the decision of one or both parents to walk away from the marriage were wrong, despite the collateral damage inevitably caused to the children. Just as with the adults in the situation, she drew her line in the sand in terms of what she could bear. Your seeming belief that her emotional wellbeing matters less than her father's doesn't, I'm afraid, lead me to think you are being fair. She has every right to decide that something she clearly finds unbearable has to end. Is it fair, when her mother was the one applying the pressure, that the mother is not the excised parent? No, of course it's not - it's the absolute opposite. But the tragic fact is that when parental alienation happens, a child may end up having to choose to remain sane, and they will almost always choose the parent who affords them primary care.
She may reconsider when she has her own kids. Or she may just remember the whole mess as a shitstorm of horrendous proportions, and continue to demonise the non-resident parent as a result. It is simply unfair of you to expect someone whose childhood was blighted by adult choices to owe those adults Herculean emotional efforts in later life, in order to soothe the hurt of one of those adults. It is NOT fair, please don't misunderstand me, that the parent who didn't engage in these games is the one suffering now. But the alternative, in actuality, is that his daughter suffered instead at the time, every bit as badly and more. And now she will have to cope with a reality about her mother, and her own choices, that would be horribly painful, all over again. Eliot said that, "human kind cannot bear very much reality" and it's the truth, none of us really live with it. We all live with comfortable self-deceptions, to a greater or lesser extent, and for her to look back at the events that led to this severance and face her mother's actions and the more complex reality may simply be more than she can cope with. I'm also afraid that, in emotional terms, it may not even be the best thing for her, if there is huge hostility between the parents. The OP's situation is a bit different, in that the mother has altered her stance and therefore the elder girl wouldn't be piggy in the middle anymore, but as I said, she would have to face some realities she probably will never want to.
It's just not reasonable to expect the children, even as adults, to behave with flawless fairness and insight and maturity and reason, when the adults responsible for their wellbeing couldn't manage it. They're going to be damaged by their experiences, these kids. Asking that they revisit events horrible enough to make losing a parent preferable, after they have presumably found a modus vivendi that allows them to move on, isn't really thinking of their best interests, is it? I absolutely understand that you hate seeing your partner suffer, and that it would be in his interests for her to see things his way, blame her mother, and reconcile. But that may not be in her interests, if it reopens a horrible can of worms. Sometimes, kids have to move past grim childhood experiences to have happy adult futures.
More bluntly, why are you assuming she has any responsibility to her parents in how she manages to survive the fallout of their divorce, and to reach an adult life worth having? She doesn't, I'm sorry but she doesn't. The person to blame here is whichever parent hated their ex more than they loved their child. If this girl lets her own kids down, abuses a partner, cheats or steals, then sure, that's on her. Cutting out the parent she can most easily live without as a way to survive emotionally is not.
Sorry this is long, but as someone whose father has married 4 times, and whose baby half and step siblings have had merry hell wreaked upon them, I take it with a pinch of salt when step-parents express indignation over the failure of those kids to meet their parents' needs.