As if even a child couldn't figure out that if Mama could turn Sister into a ghost, she could turn you into a ghost, too.
I think it would be terrifying for a child to know that.
That, plus the principle that if you have somebody in your life that causes a problem, the solution to the problem is killing that somebody.
Maybe the boy would someday be in a position to turn Mama into a Happy Ghost? Maybe when she's old and breaks a hip?
How true that is! I can see the conversation now - "you know mom when you got rid of my sister because it wasn't convenient for you and you didn't have much money to take care of her? Well, we're kind of in that position now, you're getting to be a drag on our free time and finances, I think it's time for you to go".
I just hope mommy's as willing to give up her own life for convenience and to save money as she was to give up someone else's.
I'm sure Sister Apple/Pig will be on hand to escort "Mom" to the Down Elevator.
"Mom always did like you best."