Hi Patticake, and welcome to this wonderful community of caring and understanding folks.
That's a tough dilemma, and I feel for you. I think there is a lot to consider. Whether it is cruel is, of course, one thing. But you should also consider the weakened state you feel you're in right now, and whether it's healthy for you to keep exposing yourself to a denier. Personally, I tend to get very triggered by someone invalidating what happened to me or my resultant (often debilitating) symptoms. People like that are people I have decided I *have* to cut out of my life, or I'll go insane. Long ago, I elected to go no-contact with my very abusive father, because he could never acknowledge what he did to me, and it made me so sick and angry. He ended up dying about a decade into our estrangement, without my knowing until after the fact. I am not regretful that things worked out that way (and haven't ever been, in the nearly 2 decades that have passed since his death). But that's just me...
Along those lines, how would you feel if she died during estrangement? If you think that might bother you, then it might be worth gritting your teeth and continuing to "be there for her" until she's gone. Certainly not a requirement, but something you might want to consider doing, depending upon whether you think you might end up being haunted by not having done so, once it's too late to change course.
I'm so sorry about your suffering and loneliness. CPTSD can really make life a nightmare. We're here to help support you