Obviously, his parents don't seem to have any sway over him at all. Nor his sisters. They appear to be normal decent people, people attached to their community, living in peace with their neighbors for many years, one would think they would be leaning on him a bit ,, or a lot, actually, to give Mrs. Murphy's family some relief, some kind of closure, of a sort , and return her body to them.
In Au law, and in Victorian law, keeping quiet about where the body is of a someone you killed is not the bargaining chip it might seem at first glance to be. It is a negative , in the overall makeup of the crimes , the additional crimes he is likely to be charged with, such as, for example, interfering with a corpse, if he moved her body to another location, which it seems he must have.. or/and some peculiar aspect of the method he used in the murder, all these things are yet to come down on Mr S. It is more likely that the VICPOL will be adding extra charges to those that already hang about his neck, not less because the body is still missing..
There seems, in respect of this long silence, a clear pointer to some hugely dysfunctional dynamic in this family that they have not been able to persuade him of his best interests, considering his situation. Of a long standing nature, or, on the other hand he might be particularly stubborn and stupid, and that is always a possibility.