Hi I just wonder, you mentioned how Hiro asked if she could have Nagi if Erika really wasn't interested in him (which in how she interacts with him makes me think that she really does have feelings for him, only she hasn't realized it yet)
You could see that Erika was shocked and stunned. It did seem like she wasn't sure what to say in that situation. Like usually when she hear things like Nagi saying that he's gotten closer with Hiro she would get excited, but here she seemed to be unsure how to react. I think that for Erika to start noticing her feelings, she might have to see Hiro try to really get together with Nagi.
I mean we could see her get a little embarrassed when Hiro said that it was thanks to her meeting Nagi that she could enjoy times like this with Hiro as a friend, which she denied a little. She did also say that they were the same when Hiro said that she wasn't interested in her fiancee, however we do know Erika is a bit interested in Nagi and cares for him and enjoys his company, though it without a doubt comes from her getting a really good impression of him before knowing he was her fiancee.
Anyway not sure yet, but if Hiro really tries to go after Nagi then both sisters (Sachi and Erika obviously) most likely will get jealous. I mean like Erika said to Nagi in chapter 27, hearing about him and Hiro and seeing them together is different. I mean if she didn't like him even a little romantically then she wouldn't have reacted like that.