It is on record that Sheila told her psychiatrist, that she thought that her twin sons were both evil, who wanted to rape her, and she told him that "SHE WANTED TO KILL BOTH OF THEM", whilst both were still very much alive. She wanted to kill them both, they both died, she killed them, like she said she wanted to, and intended to - it is irrelevant whether her psychiatrist thought she might actually kill her kids like she said she wanted to, and would do, the fact is Sheila said she intended to kill them both because in her mind they were both evil...
At the time Sheila threatened to kill the children, they had birthdays to look forward to, they had the rest of their lives to look forward to, she cut their lives short by shooting them, nobody else did, she killed them like she said that she intended to...
How utterly remarkable, that Sheila confesses to her psychiatrist that she wants to kill her children,, and nobody but Sheila herself, and her psychiatrist knew what she had spoken about, and her kids end up dead...
Fact is,even before she went on to kill them both, she never intended for them to celebrate anymore birthdays, which was / is how it turned out...