David Beresford-Redman has filed legal papers to validate the will of his murdered daughter-in-law  ... TMZ has learned.  In legal terms, David is opening a probate case.

David -- whose son Bruce Beresford-Redman is now charged with the murder of Monica -- is named in Monica's 2008 will as an executor of her estate. 

Monica's sisters have said they will challenge the validity of the will.

Under the will, David and his wife Juanita would inherit Monica's interest in a house she owns.  The remaining property goes to Bruce.