I kinda see this as a two way split of blame. On one hand Yurito needs to hone in on his maternal (paternal?) instinct which if he was listening to it, is probably saying anything from "my my, look at the time, I really must be going" to "WARNING RED FLAGS DANGER!!! GOTTA GET THE HE** OUTTA HERE!!". On the other hand, Coro should've taken a couple minutes to say something like "listen, I have a brother, but we're not close and don't talk for reasons X, Y, and Z. He looks like [physical description] and is [personality description] so if you see him, don't go near him. If he approaches you, walk away. And if he talks to you, say nothing back and don't trust a word he says, just come find me immediately". That wasn't so hard was it? Simple communication solves like 99% of problems before they become problems, but everyone's too secretive/stubborn/prideful/greedy/embarassed/ashamed/cowardly/etc. to properly communicate. Soooo, let's see over the next few chapters what Yurito and his baby end up dealing with...