I feel like Hiro gets a bad rep for really no reason. I'd say she isn't stringing MC along, but more working up to something.
We don't know much about her situation, so it's hard to judge, but we can extrapolate a few things:
She's in an arranged marriage and heir to a shrine. She seems to like shrines, given her highly unusual seal collecting hobby, so she probably doesn't mind the "has to inherit the family business" part. We also know she's competitive, highly intelligent, driven, and adventurous, not exactly the traits of someone who'd be fine with having her marriage partner selected for them and then transition into housewife mode right out of school. She's also been established as thoughtful and caring, so we can assume that going against her parent's wishes or burdening others with her problems/causing trouble for others in general doesn't come easily to her.
So, she's fully aware of MC's feelings for her, and from the looks doesn't mind it, but she's also rational and smart enough to know that returning his feelings would pull him into her whole arranged marriage mess, which we don't know the full extent of, and would be asking a lot from him. He'd likely be facing the wrath of her family, that of her fiancee, and would be expected to become the heir to their shrine as well, thereby cementing his future. And their relationship not working out could endanger the shrine's future, which we can pretty accurately assume isn't in Hiro's interest.
My theory is that she's working up to laying all this on him, but struggling with the burden she'd place on his shoulders for a "mere" relationship, hence her perceived indecisiveness or fooling around with his feelings.
My probably more accurate theory is that the author is a total hack and will completely butcher this potentially interesting character for more HOT SOCIAL EXPERIMENT EUGENICS INCEST (but they're totally not siblings despite being 100% raised as such so hentai logic dictates they totally wanna bang now like flipping a switch from "icky siblings" to "breed my womb, onii-chan~").