He watches the other man during the explanation. They all lived on Atlantis. It was their home, and they were his team, but it was more than that.

"They're your family," he hazards aloud. His found family. He remembers thinking it before, but the more he learns about John the more he sees it. It's part of the reason why John's so protective of even just the basest of facts about them. Ianto's grateful that he is sharing though. That he's been allowed a view into John's private world, as it were.