I didn't realize her mother was that awful. Her new husband (or boyfriend? I forget) seems like a somewhat decent person, I wonder what he possibly sees in her. Presumably she acts less abusively with her child by him.
Edit: nevermind that was someone else with him in the photo, wasn't looking close enough on my first read through. Looks like he has another family and he's cheating with her. So he's likely not much better than she is.
I'm guessing it's something like, real or imagined, her ex husband hurt her in some way, and she's reminded of him every time she sees their daughter's face. So all the negative emotions that makes her feel, she takes out on Himawari.
Himawari likes her dad, so he's probably not abusive like her mom is, which means whatever grudge Mom has with him is likely her own fault. She doesn't seem like the type of person that can accept responsibility for her own actions when they hurt people, she definitely has to twist it around in her head into always being someone else's fault. So it's probably something like, she did something that made him divorce her, like cheating or whatever, but she's reversed it in her head so it's all his fault. For embarrassing her by exposing her bad behaviour or something. Or some other post hoc justification so she doesn't have to acknowledge her own personality faults. And then she takes all that anger at her ex out on their daughter, because unlike him she can't fight back.
Cool to see Hiratsuka Papa being a great dad like usual. Even for other people's kids.
Really hope Sensei recovers soon. "Institutionalized" is a really scary place to be. Hopefully they can get out there quickly.