At around 13:40 in Episode 6 of 5-toubun no Hanayome ∬, Uesugi said that he needed to go to toilet, then Nino said, 'sure, catch up with us later'. Then Miku gave Nino an unfriendly look.
What makes Miku jealous? I don't find anything improper here that indicates a relationship between Uesugi and Nino. Maybe the lines in Japanese have something to do with it? Or is it due to Japanese culture?