I've noticed some deaths which I believe are from siblings after I put them in the grow out tub from my group 1 which has also had deaths while still with the parents. My group 2 has had zero deaths with parents so I'm going to put those weanlings in a separate tub and see if I get any deaths from them.

I have a theory that one of my females from group 1 is mental and is passing it on to her babies.
So if I continue to have zero deaths from group 2 I'll feed off group 1 and start another group with group 2's babies.

Hope you understood all that. But at least it appears you are not the only one.