That one immediately came to my mind, too. Horrifying.
I thought about the two year old, as well. Perhaps trying to comfort the baby with a blanket or pillow. Hypothetical, of course.
My first thought, though: why cover it up? Surely a two year old wont be charged for a tragic accident. Also, Im not sure a two year old would remember the incident if it wasnt discussed by the family, so psychologically it might not have an impact as it might for an older child.
My second thought: why cover it up? If the sibling was involved, perhaps the parents thought they might face charges of child endangerment, neglect, etc. Who knows what - if any - charges the parents could face? If the parents thought they could be charged, and as a result lose custody of their two year old, that might be their incentive to cover up.
Of course this is just one theory. (Not my primary one, either.)