I might be wrong, but I guess Agawa (unconsciously)knew that the other girls would reject MC, But now that her big sis is "willing to help", she doesn't want to share anymore (meaning she has feelings for MC). They sound like they get along well, so it's probably not the first time they share a sex friend, so it would mean that Agawa heard his first confession and just ignored it. This time she interupted him before he could confess because she doesn't want to have to give an answer.
Big sis understands that and pushes to become MC's sex friend to force Agawa to stop running away.
Her last word in not aimed at MC, but at her little sister : "If you keep waiting, someone might just steal him away from you."
I might be wrong, but I'm looking forward for the next chapter.