Sheila was a very misunderstood and misguided soul.
No Sheila was a rebel, and hated farm life. It seems she also hated June, as much as the twins. Colin felt the need to speak to Nevill about June making the boys pray.
Who wrote on the cupboard I HATE THIS PLACE, It certainly was not one of the twins.
Sheila was not a misguided nor misunderstood soul, she had made her bed, even though Nevill had tried to guide her.
He got the flat for her hoping she would change her ways, but unfotunately she was hoplessly addicted to hard drugs..
I think it has been proved that Sheila was a regular drug user, mixed with her medication anything could have happened and did.
As far as foster care is concerned, it is my understanding that Colin was the guardian of the children, and would not have the children taken away.
Whatever triggered Sheila was not influenced by the foster care suggestion.