Well common sense would say that she didn't die of natural causes. right? If she had, then the family would surely have called for help. So, that leaves an accident or murder. Since she would have also called for help if it was an accident, that leaves murder. Now, I don't like to think about this, but killing a 30-lb child is probably not that easy. Unless she was repeatedly abused for a long period of time, it's not likely that she is going to die from child abuse. She was too old to die from shaken baby syndrome, which leaves suffocation. There would be evidence of that on the pillow or the pillow would be missing.
Of course there are many other ways to kill someone, but judging by what others say about her, she was a devoted mother. So if she had killed Lisa, it would have had to be a snap thing, and that means hitting or throwing around, or strangulation, and any of those seem like they would leave evidence.