Caroline made an interesting comment when the said that, at the beginning, the relatives "wouldn't have known what was what". I think it should be looked at more closely.
The relatives would have been given the information BY the police, that Jeremy had given TO the police. There was no other source of information at that point. What would their initial reaction have been? Shock? Horror? Disbelief? Yes, all of those. Sheila had gone mad? She'd got hold of a gun and killed everyone? That's an awful lot to take on board when they didn't actually know, save from a letter she'd written Ann from hospital in March in which it said something about her being stressed, that she'd been ill. Certainly not MENTALLY ill. Still, I imagine that, at this point they accepted it because they couldn't get their heads round what had happened. I think it likely that the only words which passed between them were those of disbelief.
I believe it likely, that, at the start, any differences they had with Jeremy, would have been put aside. as we ALL would join ranks and pull together in a family crisis. We have Ann being supportive. Offering the hand of friendship. Being there for him. I remain totally convinced that had Jeremy conveyed himself to them as being 100% innocent -indeed, had he BEEN 100% innocent, they'd have believed it. I don't think they doubted what they'd been told until glimpses of Jeremy's behaviours began to suggest that everything may NOT be as it seemed.
For a start, Jeremy rejects offers of help. Did he consult the family over the funeral arrangements -which took place with unseemly haste. Was Colin consulted about what he'd like to happen to Sheila? OK, as her divorced husband he had no legal rights, but as the mother of his children, he arguably had the right to say he wanted them to be together. Jeremy, it seems, did all the arranging as if there were no other family. I think they'd have felt rejected by what he was doing.
I fully accept that everyone grieves differently. Some may appear not to grieve at all. I imagine this may have been how the relatives started to view Jeremy's behaviour. They may have seen their rejected offers of help as meaning that he didn't want them to know what he was planning. That he was deliberately keeping them out of the loop. Effectively, I think they'd have started to feel like the outsiders in their own family. It probably would have been at that point when they started to ask questions. Make comparisons. Notice that Jeremy wasn't telling the truth. In fact, it's very likely that, in the beginning, they over compensated and made allowances for him because of the circumstances, but as 'outsiders, looking in, hearing, watching, I think there would have been a gradual dawning that it wasn't Sheila who'd been responsible. I don't believe there was ever a unanimous blinding flash of revelation such as Saul experienced on the road to Damascus.