Another persona? She was smart, quick thinking, scheming, determined to get what she wanted, no matter the cost to others. Even managed to play sane so her drugs were reduced and she could carry out her plan to murder her family? Is that really how you see her, Lookout?
By the way, where did you read that she was still wanting to have another baby? Surely, that she'd been fitted with an IUD suggests otherwise. She couldn't whip it out like a Dutch cap, could she?
Sheila wasn't dumb by any stretch of the imagination. People are only commenting on the latter part of her life when her depression worsened, so naturally a certain amount of sympathy is called for. I put her in the same class of young women such as the late Diana whose behaviour wasn't dissimilar in a way. A " second " mother---attachment disorder and the problems that follow that. A half-hearted marriage to take you out of a previous situation slap bang into another one only worse because there are children to consider. The rest being history for both.
Reading about her wish for another baby possibly came from Claire Powell's book, it was a long time ago but I also remember that it had been discussed on here too, so I wasn't the only one who'd seen it.
I imagine that she'd been strongly advised to have the IUD after giving birth to the twins which had proved difficult with spending a while in hospital prior to their births. Then after the hard time that she must have had, to be greeted by her mother with the words " clever girl "--------the agony !