[With that reply, and even more so in spite of himself, Zero can feel as his temper flares. One sign of it is how his grip tightens around Ichiru's hand. Not so much that it would especially hurt, but noticeable regardless. He does well enough (for now) to keep it down.]
They know that you're their uncle, and their family. They know how they would never get the chance to even meet you back home, and how they have wanted to. Even if from your point of view that they really are more like strangers than otherwise.
[Proof enough that he can recognize there being truth within Ichiru's claim.]