According to the Internet red rice is commonly served to celebrate some big occasions like birthdays or weddings. I guess the sister heard his "confession" and was proud of his progress.
Fairly confident it's just a one-sided love kind of thing where she has no intentions to reciprocate those feelings.
Or maybe it pulls a "not until you graduate" kind of twist? I've seen it done at least more tastefully than how Coolkyou does the relationship between Lucoa and Shota lolol