No way was June ever a suitable candidate to have adopted children in the first place. Her mental state before adopting was rather fragile after she'd had a spell in hospital herself and had been treated by using ECT.
The above treatment was given, IMO because she'd suffered depression since learning that she could no longer produce the family that she'd wanted. Sadly the depression returned after the adoption of Sheila and she had to be admitted once again for another session of ECT. Who looked after baby Sheila at the time is not known.
In Colin's book he wrote about Sheila's recollections as a child, told to him by Sheila herself, where at one time she'd been left in the garden, presumably in her pram, to cry for hours. Sheila had realised there'd been " something different " about her mother when attending primary school, looking at other children's mothers in jeans and casual clothes whereas June was in tweeds and brogues.
I've never read that either June or Sheila had a mother/ daughter relationship, nor Sheila's interest in her mother's cooking/ baking, unlike Jeremy who'd loved baking/ cake-making with his mother, and also the shared interests with Nevill too.
Many knew of the strained relationship that Sheila had with June, including Dr. Ferguson who wouldn't have shown too much surprise had he learned that Sheila had killed her mother.
Because of Sheila's miserable start to life and once she'd been told about her adoption she set about tracking down her birth mother, who she eventually found and as Colin had stated Sheila had been the happiest she'd ever been while sharing time with her birth mother and to Sheila there was no comparison between June and her real mother.
How sad that this meeting came too late to save Sheila.