I see a lot of people complaining about the uncle and while I'm inclined to write him off as trash as the rest of the family I think he was the opposite of Hiroyuki while having the same goal. The uncle, having been raised as a successor and remained inside the family he must have felt dread. Like his hands were tied and that there was no changing the family. That's why, I think, Kohaku mentioned the uncle didn't eat much and wanted "to end their generation". I think the only way out was death for everyone. But Hiroyuki came, who was, for all matters, an outsider. And maybe there was some hope and that's why he appointed him as the master. It doesn't absolve him really, because he didn't really moved a finger except appoint someone at his death. He was a coward and left all the dirty work to Hiroyuki. But I can understand somehow. It's kinda like cycle breakers. The uncle was able to recognise the abuse but did nothing as he was tied to the cycle so he dumped it on Hiroyuki who he knew also saw the abuse
The Ritual