out of context image :)

gay ass

So he's an idiot, cute, and your lover, but your older sister?

She's*

onii-chan means brother in japanese, despite the suffix "-chan"
onee-chan means sister in Japanese
onii-chan, nii-chan, and onii-san all mean brother
onee-chan, nee-chan, and onee-san mean sister

https://i.imgflip.com/6lt8bm.gif

HAHAHAA

Ah my bad.

actually:
"brother is a cute idiot!!!" is basically what he's saying, but "senpai" cannot mean anything in this scene. "senpai" is typically reserved for senior students and aren't used for family members in the Japanese system.