OP, I do think you have been manipulated and treated with absolute contempt to the point that you can no longer separate reality from fantasy, and you need outside independent help for this. A complete stranger who no one in your family knows, and you should see that ‘ help’ alone.
There is bad spirit moving through your family. Probably from way back, but we know your husband was abused. He went on to cut your brother out, and together with your daughter, turn against your son and DIL, who it seems have done nothing wrong. Your daughter is now the manipulator, and will most likely transfer it to her child/children.
It’s all about money, and your daughter is just biding her time I’m afraid. Your husband was a good ‘ teacher’ by the sounds of it.
Please get neutral help with this before you lose everything. It sounds like your son is happy and settled with a lovely wife and family. Why on earth would he risk all that to see people who, in his eyes, have spent so much time and energy stopping him getting things.?
I also don’t understand how he is in this position, if he was an executor of his dad’s Will. There are legal obligations here, so he should have been involved.
It almost sounds like you’ve been spectating all these years. Now it’s time for you to take part, to ensure this doesn’t carry on after you’ve died.
Put it right with your son now.