Tbh less fic writers completely forgetting about Kyouka's existence when Akutagawa is staying at Atsushi's house and more Kyouka just... Being 14 and honestly not wanting to deal with the ordeal of his brother's very fucked up love life.

What I mean is Kyouka late at night bothered by the noise opening the door of her room to find a bloodied and butchered Akutagawa on the verge of passing out, Atsushi frenetically trying to hold him up and simultaneously keep him quiet, both freezing in place as soon as they see her; Kyouka looking at Atsushi, looking at Akutagawa half fainted in Atsushi's arms, looking back at Atsushi, mentally going “that's not my shit to give” and announcing she'll be spending the night at Yosano's / Kenji's / Lucy's