I've not been part of this discussion but, in general on WS, you can certainly express opinions or speculate about evidence, timelines, what police mean in their statements, and so forth.
With regards to individuals, ie actual living humans who are out there in the world living their own lives, there are two important rules: Firstly, you can't suggest that someone, who hasn't been named as a suspect by LE, is or should be a suspect. It's not our job to point fingers at people who police haven't named or dropped heavy hints about. If the police are dropping hints, I tend to drop hints too, rather than outright accuse.
The other important rule concerns victims. The family of victims are also victims, they are deeply suffering, so it is especially not allowed to start online harassment by accusing them. It doesn't mean you have to swear they are not involved, it just means you don't go there unless LE go there first.
There are things you can post about if you believe a family member might be involved in a case: you can analyse other theories for holes, you can try to construct the exact timeline, and so on. But I think it's important to remember no-one on the internet really know who has or hasn't committed a crime, and I usually just sit on my hands and wait for developments.
JMO