Alert: there's a mistranslation in this chapter that changes the meaning of a lot of things.
夜が明ける does not mean "sun sets", but the opposite, "dawn breaks".
That is, the moment of tragedy took place at dawn when the sun is rising, not dusk when the sun is setting.
This means that the Shinazugawa family stayed awake the whole night waiting for their mother and brother (Sanemi) to return.
This likely means that Sanemi spent the night searching for their mother, but had to go back home to defend his home from the rampaging demon.
This is the reason why Genya found it too dark to see what was attacking them ... makes one wonder if Sanemi and their mother knew who they were actually attacking.
And this is the reason why, despite Sanemi's knife not having any blood, let alone not a Nichirin Blade, their mother was dying in front of them: the rising sun killed her.
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The last point was something I picked up from Chinese comments: even though Sanemi had blood on him, his knife is clean.
Makes one wonder what happened between Genya trying to save his siblings and Genya running into Sanemi.