The thing is though,Jeremy had no idea of his sister's illness or even what she was capable of,which meant that he hadn't known what the psychiatrist had told CC,or he'd have really had a field day.It was only after,while in prison that he got to find out about her illness,through information which was gathered together for his first appeal.
As he told Eric Allison,he had no idea,nor did his parents ( although I imagine they kept a lot to themselves ) realise the extent of her illness and how she'd deteriorated in the 6 months leading up to the murders.
Jeremy hadn't known enough about Sheila to even contemplate placing any blame on her initially,so naturally it wouldn't sink in that she'd been so ill and he hadn't known,as it's been said that he first thought it was the raid team who shot his family.He WASN'T putting the blame on Sheila at all. Being innocent,he wouldn't have known what to think,as none of us would in the same position.