I believe that the Mom did it; most likely by accident, but she sure has tried to cover her tracks. I don’t think she’s nuts; I think she’s selfish & represents evil in human form.
I believe the Grandparents are also complicit in a cover-up. Granted, they lost their Granddaughter and must’ve known it in their BONES and they were protecting their daughter, which is usually instinctual, but wrong on every level.
All three need to be held accountable, and I pray to God that they are.