From what I gathered on the subject, Ollie kept Dinah out of the loop so her grief and angst towards Sin's "death" would look geniune... because apparently Dinah's not a good enough actor to fake it for whatever would-be kid-hunting villains that were keeping tabs on the matter.
I also love how the article points the blame of the Sin mess fallout on the *character* instead of the editorial mandate that caused it in the first place. Very professional.