@urtoast same here, the main issue is probably because the kid is drawn to face eldest sister, bubble doesn't fit her, and yet she is the one asking/stating love-interest if she is close to mc, while using a nickname that hadnt been used before, and mc name which noone remembers. Followed by the faces on next panel.
But a quick explanation would be that youngest sister says that to love-interest because apparently she is also in love with mc (hence the pouting), which is why eldest sister gets so hyped (and says it will be interesting, even though that non-bubble looks like it comes from love-interest), and love-interest looks like she is thinking "uh oh" while cold-sweating and wielding a skewed smile.