The catathymic disorder is interesting but if Sheila suffered from it one wonders why this had not manifested itself at an earlier stage with Colin,and she never had a bad word to say about Ralph(Nevill). I'm also sceptical of Jeremy's claim that there had been a discussion on the Tuesday evening about fostering the twins,firstly because I don't see when there was a time when the whole family was together without the twins and more importantly because Colin and Sheila were granted joint custody(which in practice transpired as Colin doing most of the upbringing) and the twins were the light of Colin's life and he would not have countenanced such an idea and neither would June. I do think Sheila was a schizophrenic who was liable to relapses,but as I say she had never hurt anyone else other than herself. I don't necessarily see the two illnesses as mutually exclusive,but then I think again of what Sheila said about loving her children,and far from her being an adoptee being a disadvantage in this regard it made her want to be a better mother in a way June was not to her,though due to her illness and the side effects of the medication it made that tough.
You speak as if what may have transpired that night is an immediacy. I think, for numerous reasons, the twins welfare was probably a constant source of concern to Nevill and June and was spoken of often, with or without Sheila being present. At the end of the day, it was always more about Sheila than it was about the twins because their welfare and safety was dependent on her stability. I imagine that if Sheila had told her parents that Colin had found someone else and there was no hope of reconciliation with her, they would have viewed it with mixed feelings.
Deep down, whilst he had been able to share some of the responsibility for Sheila, I don't believe they saw a sometimes penniless, bohemian sculptor with dubious morals and questionable religious beliefs as being a suitable guardian, long term, for their grandchildren and may well have viewed his total separation from Sheila as a means of gaining more control of their care in the belief that Sheila wouldn't be able to cope with them indefinitely without support, or even, that at some point, she may yet again, need to be hospitalized. This eventuality may well have already been discussed and delivered to Sheila that evening as a fait accompli and could have been worded in such a way that the children would not have been deemed to have been disturbed by it. I feel certain, that had Colin contested whatever decision the Bambers came to, regarding his sons, they would have presented a strong enough case for a court to have found in their favour.
You have, on numerous occasions said that Sheila loved her boys and would never do anything to hurt them. I believe this to be true, but THAT Sheila can't be confused with the Sheila she became during an episode. The two personalities may both have inhabitated her mind, but it's highly unlikely that they recognized or were responsible for each other.
I wonder how it may have felt to be Sheila at the start of that visit. The estranged husband, who may have shielded her from the worst aspects of her parents control, has told her of his new love. There will be no reconciliation. Does she find herself faced with being in a situation in which she and her boys are trapped. She couldn't have been unaware that the freedoms they enjoyed in London would be severely restricted if they were forced to move back to the farm but she may have been aware that she couldn't manage without support.I feel it would be reasonable to assume that she felt hopeless.