Ruth is told by her FiL that they will be supervising her with the baby until he is old enough to go to school, and I think he intended that school to be a boarding one.
She can't stand the thought of it so she rigs an electric wire up to the pram and parks it at the very end of the strip of grass she was cutting in her garden, then goes inside to hide.
When her MiL arrives she watches to see what she does, deciding that if she goes to the pram first she will click the switch to make the wire live, if she comes in the house first she will accept what her FiL has just said. She keeps saying to herself "it's up to her to decide" if she lives or dies by her actions.
MiL goes straight to the pram and is electocuted, DiL moves the pram, reconnects the wire to the lawnmower and puts the lawnmower in place, then calls for help and lies, saying MiL just decided to take over cutting the grass and got hurt.
I left the last couple of pages unread, just in case I decide to read the book one day, so I don't know if she was caught or got away with it and out of context like that it's hard to know if MiL or DiL or both were crazy and to blame.
It's still far fetched but I prefered it to the TV ending.