Jamie doesn't know what to say when he first meets the shell that is his sister, his sister with someone else in her body.
He considers being cruel, like Jared.
He considers being kind, like Jeb.
He can see the merits of both.
He doesn't know what he feels towards this thing, because he doubts it's actually the creature within her's fault that she is here.
He's seen people who have been fully taken over by the Souls, those that betray their own kin - like their father. Sure the Soul (Melanie) is here, but she doesn't seem to have any wish to escape and report them to the Seekers. In fact, she just seemed to want to see the rest of them were safe, and then give up.
Jamie can tell that she's waiting to die.
And when she's wearing his sister's skin, that's the scariest thing he's ever seen.