He'd been scared that she might not have reacted well to him. Doubt had formed over the course of their time apart and Pietro had come to blame himself for not acting faster or thinking better on his feet. He'd watched a police man take his sister away and had then spent fruitless years searching. He'd understand if she was angry at him for this. Maybe she'd spent their time away coming to resent him, to wonder if he'd abandoned her. It was a natural thing to assume given enough time and Pietro, who could think up any number of different scenarios about his sister, couldn't blame her for anything she might have felt.
He hugged her tightly, wrapping his arms around her for what was probably too long, but not especially caring. He'd missed her so much. "You're not an idiot," he said and almost in the same breath, he added, "I missed you."
Perhaps when the shock of it wore off, he'd feel more emotions aside from the flooding relief that she was alive and okay and here.