There's about two volumes of secrets from the source material getting spilled here all at once. I dunno if there's an incoming axe or they just wanted to jump ahead to later events.
From what I can gather from the comic-walker website for this manga... there's indeed one more volume's worth of chapters at least, up to chapter 26. But in the chapter list there's a headline titled "notice of suspension of publication" (according to Google Translate, I can't read Japanese). Opening it doesn't really provide much more clear info though: "I'm going to take a good night to work on the book... Hot spring because it's winter! Kina Oyama" (there's a hot spring pic with that text). Again that's Google Translate, so I dunno if it just means it's a temporary hiatus or something else.
So we already knew that she lives alone and has been alone for a while. We also know that there's something about the actress in the drama that bothers her. However, the actress being her mother, and her mother being abusive, those are both new in this chapter. But with finding out that Nikaido is born out of wedlock, it does kind of explain both her family situation and her mum's treatment of her.
Being born out of wedlock does not explain her mother's attitude. Her treatment would be considered abusive in most occidental countries, even in this situation. I suspect she would have been abusive even within a marriage, as I suspect she probably only saw a child as an impediment to her career. It is however possible that she's the child of a rape.
Well I'm assuming that Haruki was born out of wedlock as an unplanned child, so her mum resents her for being born. I'm not defending or justifying her actions by any means, simply saying that with the situation as we currently understand it, there is potentially a reason for her mother's actions. A bad reason is still a reason.
