What is the ruling on not fulfilling a father's will to buy his house after his death and distribute the sisters' share, due to inability and unwillingness?

A parent's will is recommended to be fulfilled, but it is not obligatory. The execution of their covenants after their death might refer to the child carrying out covenants that the parents had committed to but were unable to fulfill, or fulfilling promises that they entrusted the child to carry out after their death. If what they enjoined was obligatory, then its execution is obligatory. If it was recommended, then its execution is recommended. There is no blame if the son is unable to do so.