Kaguya-sama wa Kokurasetai: Tensai-tachi no Renai Zunousen - Vol. 19 Ch. 185 - Ai Hayasaka's Friends, Part 4

[MangaDex]

 

[danvolodar]

I still don't understand the logic here. When Kaguya's brother held her maid, accusing her of treachery, wasn't the only expected answer from her "Thank you, dear brother, I owe you one, this matter will be further settled internally between her and me"?
I mean, Kaguya was bound to learn of Ai's whereabouts sooner or later, anyway, and treacherous or not, she was still her retainer to deal with?