[Not really sure what to say, honestly. He's bad at comfort, in general, and anything he could say would probably have already occurred to Kamui - that Sei obviously isn't going to stay dead, it's natural for Subaru to be upset (understatement), and many other things. And mostly that all this probably just confirms a lot of things for Kamui, in regards to their brothers.]
[...] [So just shifts the hand on his elbow to splay it across his back instead, just touching him, doing nothing more.]