Let's not forget in all this that June herself had suffered severe depression in 1959 which resulted in her having ECT therapy,then again in 1983. Sheila had been adopted in 1958,so a bad start for a child who was also to suffer later on,and whose mother's condition was pretty delicate to face the ups and downs of bringing up a child.
I think there was a battle between the two women on the night of the murders.