You are going to need someone review the Wills. By your query it would appear that the great-grandfather left his wife a life-estate in the realty with various other persons being the remaindermen. If that is the case, the wife would not be able to devise her interest in the realty to anyone in her will because her interest terminates when she dies.
If the great-grandmother is the wife, the power of attorney granted to her daughter no longer has any effect because that terminates upon her death as well. However the daughter, as executor, does have a duty to protect the property of the great-grandmother's estate.
There isn't very much information in your query about the deed that the son claims to have, but if great-grandmother only had a life estate in the land, then all she could deed over would be an interest in the land during her life time. When she died the land went to the remaindermen.
Then, just to make things more complicated, you are going to have issues of dower.
Sounds like this will be in court, and anyone who wants to claim an interest in the property is going to need an attorney because it's only going to get more complicated from here.