Absolutely. It's backed up by Colin, who said his ex-wife didn't speak a word on the drive down to White House Farm. In that state is it any wonder she had no idea in the few seconds it took for him to wake his sister from her slumber and lead her across the landing to her demise?
Maybe she was angry with Colin?
So if she was that bad why leave her with the children ?
It was in one of the last books , which I am sure you have read , about different witnesses at the party and the preceding days giving different accounts of her behaviour.
Or perhaps she was heading for an episode ? Perhaps just depressed rather than zombie like ?
To say it is why Jeremy chose that day is no different to saying that is why she could have committed the murders.