The fact that they thought he killed everyone might have had something to do it it also.
That doesn't excuse their actions or behaviour. The police and pathologist knew Sheila had killed everyone. The police were so convinced regarding the original evidence, that they expressed disgust towards the relatives' insinuations. Funny that - since we are constantly told on here that there is no evidence. So the police at the time were convinced to the extent that they were disgusted at the relatives insinuations - purely based on no evidence??
The relatives were told some of the reasons as to why the police held Sheila responsible, at a time when there was no need for police to conceal certain evidence: as they themselves were not actively pursuing JB as a suspect at that point, there was far less motivation to conceal any known evidence that linked Sheila. Whatever the relatives were told, it was enough to convince two gun experts among them, that Sheila could have been responsible.
The problem with that scenario - is that it did not resolve the self-serving concerns and fears that the relatives harboured.