I think Dr Jem deliberately broke the bad news to the Balbini twins in such a way because he was angry with them for kidnapping Sybil.
And that would be an interesting and 'difficult' psychological scene if there were any sense that EBD thought this was a 'good' character behaving badly in the heat of the moment, out of anger and relief.
But there's no hint at all that she thinks he's other than 100% right to break the news of their mother's death to a pair of young children by painting a cruel picture of her dying moments as spent waiting in vain for them, and telling them (I think) that their punishment will be that they will always know this.
I think EBD does actually think this is a fitting punishment/poetic justice -- that the children have 'brought it on themselves'.