Hi velaine and welcome to GN. It's always so sad to hear from yet another estranged parent.
So the years of egg shell walking haven't saved you from this heartache; I'm so very sorry but hope that you'll be able with time, to get some comfort from the knowledge that you did everything you could to prevent this from happening and the this end result is not of your making.
It's shocking to see the AC we love, who was once so happy, loving and caring become someone we hardly recognise and when this does happen, it's often due to their partner who uses coercive control to manipulate and control.
They often reject their entire family. Our son has done the same apart from his brother but I do believe if it were not for the fact that he lives so far away in Australia, he'd have been estranged too.
"Is it likely that things will change"? I'm sorry but it does sound unlikely especially as you've been aware to a certain extent the nature of their relationship, which explains why you've been walking on egg shells.
You cannot harden your hearts, no matter how hard you try. If and when that happens, it happens over time and even a hardened heart continues to love the child whose hurt us so badly.
We've been estranged for more that 8 years from our son and only GC. There is life after estrangement and with time the tears are shed less frequently and the heartache which never goes away, does diminish too.
I don't think the why's ever go away either, it's just that you don't keep asking because there are no answers to be had.
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It may help you to post on the support thread on this forum and/or if you think it may help, please feel free to private message me.