Aniki no Kanojo ni Naru, Onnanoko ni Nacchatta Otouto. - Ch. 307

[MangaDex]

 

[xanon]

These two...there is no way they didn't take an oath that if either of them turned into a girl, they'd let the other one hit it.

 

[Flame-2-Haze]

She wants to what?

 

[kuma]

wait, how old is MC? mister?

 

[WhimsiCat]

wait, how old is MC? mister?

Pretty sure they're the same age.

My guess is she's using some unusual pronoun because otaku speech.
Something like 'danna'

 

[CrowGoesCaw]

I bet she's dressing like that for him but is too shy to admit it