Winding up the estate according to an unislamic will

Q: What should be done if a person dies with an un-Islamic will? The will specifies its own heirs and the shares for each person with no Islamic principles adopted. Non-heirs, according to Islam, are included also. What should be done in this case?

What about the the 1/3 available for bequest. Can the Islamic heirs agree on how to distribute this on their own through mutual agreement?