May I make a slight comment here on behalf of "next wives" everywhere? Keep in mind that the big pay off at the end of it just might be severely diminished by the fact that the OP's father is in his high eighties and is showing signs of dementia. There might not be any estate to worry about if he requires extensive late in life dementia care. Elder care is not only burdensome, it is very expensive. If there were no younger stepmother to drive him around, live with him and provide/oversee care for him, it would be the responsibility of the others involved, his children. If he were to require companions, caregivers, this would cost a lot of money. If he were to get in the condition that he requires actual care in a nursing or memory care facility, that property, those assets, could melt like snow in the desert.
A big responsibility that a spouse, especially a younger spouse assumes is the care of a person who may live in the course of a lifetime to be very old and require a lot of care. Expensive care. That's one of the aspects of marrying an older person than yourself involves. If she has stayed with him for twenty years, she has probably enhanced the quality of his final years, and that is what he is funding. If she had not been here, the children might have found themselves with many more responsibilities. I hope he has taken steps to arrange things so that she is well taken care of during the rest of her life as well.