-In a Japanese family, the mother plays a much bigger role in the family dynamic. I think you're using American standards to judge her situations when you should be thinking about Japanese cultural norms.
-It's not easy to just stop liking someone.
-I'm sure she's well aware of their support, but the whole problem (her illegitimacy) isn't something she can change. That's why the situation is more complicated than you make it out to be.
-It's hard to give a fuck about other people's situations when it feels like no one cares about your own. I know Shin and Shin's dad are trying their best, but Japanese men aren't really socially equipped to provide her with the support she needs. I would expect Shin to give her the most support, but he has his own stuff to deal with.
Anyway, my point is that Hiromi might not necessarily be a shining angel of light who's been damned by God, but you have to understand where she's coming from.