You can't argue with facts Steve. The facts are (1) that Colin had told Sheila that very night that their relationship was over. (2) Sheila hoped they would get back together. Those facts cannot be denied. Another fact if you will receive it is that she referred to her children one time as if the devil had possessed them.
Marked in red: Maybe her fear was that now Colin had another love in his life that they may get married and that Colin maywant to take them away? That is a very real fear with single mothers as well?
The incident you refer to about Sheila seeing the Devil in her twins was in her first stay at St. Andrews' in September 1982. Though nobody would say Sheila was fully compos mentis by the end of her second stay in hospital in March 1985 she no longer talked of the twins in that way,but did have a religious fervour,which was picked up by Jeremy when he led her into the master bedroom that Wednesday morning like a lamb to the slaughter and shot her quickly under the chin unawares.
Of course Colin not wanting a reconciliation was a blow to Sheila,but she had had time to accept this by the August as a fait accompli. It doesn't make Sheila a schemer just because she knows that some day she will inherit from her parents,but it does give someone a sense of security and more of a reason to live.