Board Thread:Questions and Answers/@comment-24098614-20180105023325/@comment-36009152-20190821131352

2404:3C00:21B0:E2D1:A831:5C83:76F9:7E91 wrote: when they were on the hyakuya family that doesn't mean really that they are genetically brothers but just like Family Sure, they are an orphanage family, like the children there adopt the surname "Hyakuya", Yu and Mika are not blood related.

As you point out in their early age, and also the time in between, to their current connection, it is unwavering and deep!