Should the children of the deceased relative, to whom his wife granted a plot of land, pay its price to the daughters of the deceased's nephew who suffers from a mental illness, or should they pay all the heirs?

If the land gifted to the husband's relative has been built upon and is part of the inheritance, then this construction is on land usurped from the mentally incapacitated heir. The heirs of this person are not obligated to accept the value of the land. Rather, they have the choice between demanding the demolition of the building and the return of the land to them, along with compensation for removal (rent for the period of the building's presence) and any decrease in value, or paying the value of the building as scrap to the heirs of the usurper and taking possession of both the building and the land. It is best to refer the matter to the Sharia court for consideration.