From The Murders At White House Farm Chapter 15:
Painting the exterior walls at Vaulty that afternoon, Michael Horsnell noticed that all wasn't well. At quarter past three he recognized June's silver Renault as it turned in at the drive. He watched June, Sheila and the twins head into the garden: "Mrs. Bamber was playing with the two boys, jumping over small hedges and running up and down the garden. Sheila was with them, but she was like a zombie. She walked very rigidly, and the only part of her that moved was from the knees down. She didn't even turn her head from left to right. I did not hear Sheila speak at all. They went into the house."
I can fully understand Sheila's state at that time which was a few hours before the tragedy but nevertheless warning signs from someone who was about to change the lives of everyone in a split second.
The moods of anyone in such a state are a fore-runner of how their minds are going to react depending on how the rest of the time pans out. It wasn't a pleasant evening over dinner by all accounts and as the psychiatrist had already said that when a certain point in the conversation had been mentioned would have " tipped her over the edge ". The fostering subject as well as Colin's suggestion of 100% care of the twins.
Any mother, I don't care who it is would see red, be upset and display disapproval but because Sheila had been in no state for such a conversation her irrationality took over.
In a way similar to when JM put the pillow over JB's face, but it was the words that she'd said which rang familiar " if I can't have you nobody else will ", which would have been in Sheila's thoughts at that time of the murders.