Don't know why they're so surprised. This woman porked him, received his confession, and responded with "Nah, you're like a little brother, but you fuck well so I'll keep up with that."
Also, this has literally been the arrangement up to this point. Souta has been relieving the girls' urges the whole time, even after he started dating Captain. Why would someone simply mentioning the sister's possible feelings for Souta make a difference? The rest of the team are probably going to keep riding him, too. Souta's heart is up for grabs, but his dong is community property.