Spoiler: Sister is a type that exploits people around and doesn't do jack shit in house. If there's nobody who would do basic stuff behind parents backs, they would throw her out.What is the reason of “sister breaks with boyfriend - brother should go back home”? She can’t live by her own without worrying or bothering him?
She's timid, her reaction in stress is "sensei mode" in which everything has to be reasonable as Yamato taking care of his sister after break up seems necessary to her (she doesn't know she's abusive) and there's still pretty taboo subject with teacher-ex student with 10 year age gap.No, it's not all "okay". The fact that she needed him to come back and return the key to even say she didn't want him to go is the very definition of "not okay".