The top and bottom of Sheila's thoughts latterly was that she'd known she was going to lose the twins either to Colin, foster carers or some other means and the way her mind must have been working by losing custody/ parental control to someone else was more than she could stand and in such situations there have been mothers who have killed their children for this reason alone who haven't necessarily had any known mental health issues.
Because Sheila did have issues, her thoughts would have spurred her on. Both her parents probably tried to tell her it was for the best for the twins as fostering had been discussed at suppertime and no doubt after JB had left, the discussion had become heated.