Now that we have a clearer idea of why the Shinmother is reacting as she is, after episode 6, can we forgive her a bit more? She is not reacting well to the situation, but it is a terrible situation to have to deal with.
To her, Hiromi is a living embodiment of her own failure, rejection, and shame, a constant reminder of her husband's unfaithfulness to her. And her husband demands that she treat Hiromi as her own. That would be nice, but it isn't so easy.
In any case, Takahashi Rieko is doing a great job with the character. I've posted a bit about Takahashi-san over in the voice-acting thread