Corrected the first part TL. Kouhai is saying Osananajimi-chan that they should practice for when Osananajimi-chan gets confessed by Senpai (Onii-chan). 「先輩に告られた」(Confessed to by Senpai).
It's also why Osananajimi-chan reacted like that in the third and fourth panels (she's acting like she got confessed by the onii-chan).
Eeeh~!?*BAM!*Me too...... This whole timeー
You've been the only one......She got seriously into thisHupーEh!?I've always loved you!
Become my woman!!ーAah
Onii-san is... to me...A-Ah, well...Hey, let's practice for when you get confessed to by Senpai!