Turns out you're not responsible for other people's emotions. Iku falling in love is not Misaki's fault, nor is it her responsibility. She could have done a better job of responding to that, but that is unfortunately common to humans, and it's ultimately Iku's responsibility to manage his own feelings.
As for "hitting on" Iku, I could easily be wrong here but I think that whatever phrase was used here is one that is frequently translated as "flirting with", and the probability is high that when she said it, she was doing so in a teasing/playful way rather than flat-out stating that she is a criminal predator. Does that leave things open for misinterpretation? Yep. Is that a common problem in human communication? Yeeeep.
So yeah, sucks that she was a young person just trying to be friendly/companionable with a kid who initially looked lonely and who later she found out was going to be her brother-in-law rather than being a trained child psychologist who always performs every human interaction properly and flawlessly. But please, do let me know if you ever meet anyone who matches the latter description. Bigfoot would like a friend.