Boyfriend/Brother?

[Yoshioko]

I called Rinko during Phys. Ed., "What are you doing," and at the end of her response, she says, "You don't have to worry Big Brother♥"

Big Brother?
Understandable before confession, but after?

Is there some sort of Japanese cultural context in which the
title "Big Brother" is also boyfriend, husband, or something?

[MorisatoK]

She's just teasing you.

This happens in one of the dialogues in friend mode and sometimes in lover mode (probably depending on character type).

Of course it happens more often (read: always) in lover mode if you tell her to call you oniichan (and she agrees).