Cultural Context

[BoyfriendBrother]

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?

[Yoshioko]

I forgot to sign in when I posted this one.
Delete it if someone could.

I'm reposting it, keep the newer one.