Family is something that Kain takes with the utmost seriousness, even though he technically doesn't have any blood relatives still around, himself. He gives her a questioning look when she mentions

"It must be nice... to have such a large family, so many others to always rely on. Though the sibling rivalry would be terrible, twins or otherwise." He could only imagine what it would have been like, had he and Cecil been actual blood brothers. It was bad enough being pseudo-adopted brothers of a sort, though all the rivalrous feelings had been Kain's alone. "I'm also an only child... though in part I grew up with someone who was essentially a brother to me... and though they're not family, the comrades I've traveled and done battle beside... are close to it."