Roch, in the world of human nature, whilst ANYTHING may be possible, it isn't necessarily probable.
Hi Jane.. my personal opinion.. I'm sure others will disagree.
It seems probable to me that Sheila killed her children and attempted to commit suicide, as a result of the following combined triggers:
(1) Unresolved internal conflict, stemming from the ingrained religiosity of her mother, which fed in to her psychotic delusions / self-perception
(2) Reduced dosage of anti-psychotic medication
(3) Recent contact with her birth mother and any resulting, conflicting emotions this would inevitably stir-up
(4) Resentment towards Colin Caffell for obvious reasons, including rejection and the annulment of any rekindling of their marriage
(5) Any suggestion that she required extra assistance with the care of the twins – which could be interpreted as interference or loss of control over them (and have a potentially catastrophic effect!)
(6) Culmination of any historic suicidal ideation, due to dissatisfaction with her problematic life
With regard to any interpretation of ‘temporary fostering’ being irrelevant (because Colin would have care of the kids etc.) – do people really believe that Sheila would be happy for Colin to increase his responsibility for the kids to the detriment of her own responsibility?