[There might be some sort of weird not-actually-empathetic empathy at work here. It makes Kaoru think about the time spent before Chizuru had arrived and it's a stark, unpleasant reminder that she might be taken away at any time. It would be too difficult to handle, so it's out of respect for that difficulty that Kaoru leaves the topic largely untouched.

It's too private, and it would be too painful. Both for Tenka, surely, who loves his family so much but keeps to himself so closely that he doesn't even openly grieve the loss of a sibling, and for Kaoru, who just fails at sibling-related emoting in general.

Tenka's statement is echoed, either way, with a soft-sounding:]

It's the job of the elder sibling to look out for and protect the younger ones. [Definite agreement, there.] And Nishiki-san will likely benefit from having someone to watch over her, too.