Scipio
he told Julie of his plans so I am told the explanation given by Jane seems credible to me.
He told Julie well in advance. He did so before he was sure he was even going to do it. He complained to her about them. He needed someone to bitch to and she was his choice. She didn't blab and he even thought she was trying to help him with the sleeping pills so he used her as his confidant and thus kept her appraised of his plans.
That is a big difference between planning everything out and deciding to try confiding his intentions in his sister when he had no reason to and she would be able to ruin everything. He didn't need Sheila's help, he didn't have a confidant type relationship with her where confiding in her would provide him comfort and have any other reason that would benefit him from telling her.
It's no more believable than the police approaching the family and asking them to agree to help lie about finding the moderator. The situations are analogous. Police had no need to recruit the family for such and the family could end up ratting them out intentionally or unintentionally and thus could ruin everything.