// Pseudocode:
TPerson = class
private
  OnDblClickForFamilyTree: TNotifyEvent;
public
  property _OnDblClickForFamilyTree: TNotifyEvent read F_OnDblClickForFamilyTree write OnDblClickForFamilyTree;
  procedure DblClick; override;
end;

procedure TPerson.DblClick;
begin
  inherited;
  if Assigned(F_OnDblClickForFamilyTree) then
  begin
    F_OnDblClickForFamilyTree;
  end;
end;

TFamilyTree.Somewhere;
begin
  SomePerson._OnDblClickForFamilyTree := PersonDblClicked;
end;

procedure TFamilyTree.PersonDblClicked(Sender: TObject);
begin
  makeProband(Sender as TPerson);
end;